Please refer to following Oracle Note:
+ How to Change Applications Passwords using Applications Schema Password Change Utility (FNDCPASS or AFPASSWD) (Doc ID 437260.1)
Assuming that you have changed password using alter command and haven't tried any other options like FNDCPASS to revert.
In that case, MOS Notes Doc 566127.1 , provided by Srini is the ideal solution.
Hope this helps!!!