If you have lost your sysman password or don’t know the current sysman password, follow below steps

Stop oms

[oracle@oel6oem dbs]$ $OMS_HOME/bin/emctl stop oms
Oracle Enterprise Manager Cloud Control 12c Release 5
Copyright (c) 1996, 2015 Oracle Corporation.  All rights reserved.
Stopping WebTier...
WebTier Successfully Stopped
Stopping Oracle Management Server...
Oracle Management Server Successfully Stopped
Oracle Management Server is Down

Run below password change command from oms home if you do not know the password. you need to provide your sys password (system) and new password for sysman (sysman123).

[oracle@oel6oem dbs]$ cd $OMS_HOME/bin
[oracle@oel6oem bin]$ pwd
/u01/app/oracle/oms12cr5/oms/bin
[oracle@oel6oem bin]$ ./emctl config oms -change_repos_pwd -use_sys_pwd -sys_pwd system -new_pwd sysman123
Oracle Enterprise Manager Cloud Control 12c Release 5
Copyright (c) 1996, 2015 Oracle Corporation.  All rights reserved.

Changing passwords in backend ...
Passwords changed in backend successfully.
Updating repository password in Credential Store...
Successfully updated Repository password in Credential Store.
Restart all the OMSs using 'emctl stop oms -all' and 'emctl start oms'.
Successfully changed repository password.

Verify you are able to connect using this sysman password in repository database

[oracle@oel6oem bin]$ sqlplus sysman/sysman123@emrep

SQL*Plus: Release 12.1.0.2.0 Production on Sat Apr 4 15:19:22 2020

Copyright (c) 1982, 2014, Oracle.  All rights reserved.

Last Successful login time: Sat Apr 04 2020 15:15:38 +05:30

Connected to:
Oracle Database 12c Enterprise Edition Release 12.1.0.2.0 - 64bit Production
With the Partitioning, OLAP, Advanced Analytics and Real Application Testing options

SQL> exit

Stop and start oms

[oracle@oel6oem bin]$ $OMS_HOME/bin/emctl stop oms -all
Oracle Enterprise Manager Cloud Control 12c Release 5
Copyright (c) 1996, 2015 Oracle Corporation.  All rights reserved.
Stopping WebTier...
WebTier Successfully Stopped
Stopping Oracle Management Server...
Oracle Management Server Already Stopped
AdminServer Successfully Stopped
Oracle Management Server is Down
[oracle@oel6oem bin]$ $OMS_HOME/bin/emctl start oms
Oracle Enterprise Manager Cloud Control 12c Release 5
Copyright (c) 1996, 2015 Oracle Corporation.  All rights reserved.
Starting Oracle Management Server...
Starting WebTier...
WebTier Successfully Started
Oracle Management Server Successfully Started
Oracle Management Server is Up

another way to verify if new password has been changed. enter sysman password (sysman123) when prompted

[oracle@oel6oem bin]$ $OMS_HOME/bin/emctl status oms -details
Oracle Enterprise Manager Cloud Control 12c Release 5
Copyright (c) 1996, 2015 Oracle Corporation.  All rights reserved.
Enter Enterprise Manager Root (SYSMAN) Password :
Console Server Host        : oel6oem.localdomain
HTTP Console Port          : 7788
HTTPS Console Port         : 7802
HTTP Upload Port           : 4889
HTTPS Upload Port          : 4903
EM Instance Home           : /u01/app/oracle/gc_inst/em/EMGC_OMS1
OMS Log Directory Location : /u01/app/oracle/gc_inst/em/EMGC_OMS1/sysman/log
OMS is not configured with SLB or virtual hostname
Agent Upload is locked.
OMS Console is locked.
Active CA ID: 1
Console URL: https://oel6oem.localdomain:7802/em
Upload URL: https://oel6oem.localdomain:4903/empbs/upload

WLS Domain Information
Domain Name            : GCDomain
Admin Server Host      : oel6oem.localdomain
Admin Server HTTPS Port: 7102
Admin Server is RUNNING

Oracle Management Server Information
Managed Server Instance Name: EMGC_OMS1
Oracle Management Server Instance Host: oel6oem.localdomain
WebTier is Up
Oracle Management Server is Up

BI Publisher is not configured to run on this host.

If you know your sysman password but want to change it follow below steps

Stop oms

[oracle@oel6oem bin]$ $OMS_HOME/bin/emctl stop oms
Oracle Enterprise Manager Cloud Control 12c Release 5
Copyright (c) 1996, 2015 Oracle Corporation.  All rights reserved.
Stopping WebTier...
WebTier Successfully Stopped
Stopping Oracle Management Server...
Oracle Management Server Successfully Stopped
Oracle Management Server is Down
[oracle@oel6oem bin]$

change the password using below command and enter existing sysman password and new password when prompted

[oracle@oel6oem bin]$ $OMS_HOME/bin/emctl config oms -change_repos_pwd
Oracle Enterprise Manager Cloud Control 12c Release 5
Copyright (c) 1996, 2015 Oracle Corporation.  All rights reserved.
Enter Repository User's Current Password :
Enter Repository User's New Password :

Changing passwords in backend ...
Passwords changed in backend successfully.
Updating repository password in Credential Store...
Successfully updated Repository password in Credential Store.
Restart all the OMSs using 'emctl stop oms -all' and 'emctl start oms'.
Successfully changed repository password.

verify new password

[oracle@oel6oem bin]$ sqlplus sysman/system123@emrep

SQL*Plus: Release 12.1.0.2.0 Production on Sat Apr 4 15:59:11 2020

Copyright (c) 1982, 2014, Oracle.  All rights reserved.

Last Successful login time: Sat Apr 04 2020 15:56:47 +05:30

Connected to:
Oracle Database 12c Enterprise Edition Release 12.1.0.2.0 - 64bit Production
With the Partitioning, OLAP, Advanced Analytics and Real Application Testing options

SQL> exit

stop start oms

[oracle@oel6oem bin]$ $OMS_HOME/bin/emctl stop oms -all
Oracle Enterprise Manager Cloud Control 12c Release 5
Copyright (c) 1996, 2015 Oracle Corporation.  All rights reserved.
Stopping WebTier...
WebTier Successfully Stopped
Stopping Oracle Management Server...
Oracle Management Server Already Stopped
AdminServer Successfully Stopped
Oracle Management Server is Down
[oracle@oel6oem bin]$ $OMS_HOME/bin/emctl start oms
Oracle Enterprise Manager Cloud Control 12c Release 5
Copyright (c) 1996, 2015 Oracle Corporation.  All rights reserved.
Starting Oracle Management Server...
Starting WebTier...
WebTier Successfully Started
Oracle Management Server Successfully Started
Oracle Management Server is Up

verify password chnage alternatively

[oracle@oel6oem bin]$ $OMS_HOME/bin/emctl status oms -details
Oracle Enterprise Manager Cloud Control 12c Release 5
Copyright (c) 1996, 2015 Oracle Corporation.  All rights reserved.
Enter Enterprise Manager Root (SYSMAN) Password :
Console Server Host        : oel6oem.localdomain
HTTP Console Port          : 7788
HTTPS Console Port         : 7802
HTTP Upload Port           : 4889
HTTPS Upload Port          : 4903
EM Instance Home           : /u01/app/oracle/gc_inst/em/EMGC_OMS1
OMS Log Directory Location : /u01/app/oracle/gc_inst/em/EMGC_OMS1/sysman/log
OMS is not configured with SLB or virtual hostname
Agent Upload is locked.
OMS Console is locked.
Active CA ID: 1
Console URL: https://oel6oem.localdomain:7802/em
Upload URL: https://oel6oem.localdomain:4903/empbs/upload

WLS Domain Information
Domain Name            : GCDomain
Admin Server Host      : oel6oem.localdomain
Admin Server HTTPS Port: 7102
Admin Server is RUNNING

Oracle Management Server Information
Managed Server Instance Name: EMGC_OMS1
Oracle Management Server Instance Host: oel6oem.localdomain
WebTier is Up
Oracle Management Server is Up

BI Publisher is not configured to run on this host.