If you would just read the Umbrel Troubleshooting manual before anything stupid, you wouldn’t be in this situation.
Please read it and you will find there the instructions how to do it.
But anyway, why do you want to change that password?
You will use TH anyway only on your LAN (secured) or through Tor if you are outside. Also TH app is using a total different onion address than your node, so if you do not leak it to anybody, there’s NO risk that anybody can enter/use it.