Steps to change sysman password in Oracle Enterprise Manager 12c

Stop oms . Do not include '-all' as the Admin Server needs to be up during this operation.

cd /u01/app/oracle/product/MiddlewareR5/oms/bin/
[oracle@em 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


Modify the SYSMAN password:-
emctl config oms -change_repos_pwd -use_sys_pwd -sys_pwd <repository sys user password> -new_pwd <new sysman repository password>
Example:-
[oracle@em bin]$ ./emctl config oms -change_repos_pwd -use_sys_pwd -sys_pwd welcome1  -new_pwd welcome2#
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.
[oracle@em bin]$

Stop the admin server on OMS machine
[oracle@em 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

Start the OMS
[oracle@em 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

Check OMS status
[oracle@em bin]$ ./emctl status oms
Oracle Enterprise Manager Cloud Control 12c Release 5
Copyright (c) 1996, 2015 Oracle Corporation.  All rights reserved.
WebTier is Up
Oracle Management Server is Up