To reset the password you need access to the server filesystem.
1. Open the file ORACLE_HOME\j2ee\home\config\system-jazn-data.xml
2. Look for the user you want to reset the password for.The below XML sample is for the user oc4jadmin
3. Change the credentials
The password now becomes welcome1.
4. Restart the server
5.Login using the user oc4jadmin and password welcome1.
Now you will notice that after the server has started the credentials again become hashed.