SSH is just a terminal where you can interact with your node through command lines. That’s all.
So once you open a terminal on your computer and enter:
ssh -t umbrel@umbrel.local
Instead of umbrel.local also you can use the local IP of your node.
The password is your personal Umbrel login password. But you have to type it in blank (you will not see what you type) and press ENTER.
After that you start using the debug commands described in the troubleshooting manual for what you need.