Hi Sai,
I'm not sure how you got to this situation, but it's probably quite difficult to resolve, as you cannot change the password via the Cloud Portal (as the app is not running) and not via some After Startup logic (as your app won't startup).
The only fix I can think of is by changing it in the database (or just remove the user, as changing passwords isn't so straight-forward), so if you have access to the database you can do that.
If you don't have access to the database, I would suggest to download a backup, change it locally and restore the backup in the acceptance environment.
Hopefully this works, good luck!
Johan