You may lose or you may want to change SYSMAN password. So here are the steps

Step 1:
Set related env. such as ORACLE_HOME, ORACLE_SID

Step 2:
Stop all OMS processes

cd <oms home>/bin
emctl stop oms

Step 3:
Change the password from SQL*Plus

alter user sysman identified by “oracle_new_pass_here”;

Step 4:
Change the SYSMAN password

cd <oms home>/bin
emctl config oms -change_repos_pwd -use_sys_pwd -sys_pwd <sys password> -new_pwd <new sysman 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.

Step 5:
Stop the Admin Server on the primary OMS and restart OMS:

cd <oms home>/bin
emctl stop oms -all
emctl start oms

Step 5:
Check that oms is up and running:

cd <oms home>/bin
emctl status oms -details