Note: If you have an HA cluster, then you must stop all the slave nodes before following the below steps on the master. After resetting the password on the master node, the slave nodes can be restarted.
Resetting Password for FAS Versions 2.3.0+
Resetting Password for Older Versions (FAS 2.2.7 & earlier).
There is a shell script in the bin folder of the Fusion Application Server
called add-user.sh. This can be used to add new admin users & change the
password of an existing admin user. To change the password of an existing
user you simply specify the name of that user when prompted by the script.
Details can be found in the FAS admin guide, attached, in the section
titled "Controlling access to the Management interfaces". I've attached a
short screen capture that show the commands that you will need to run.