When my upstairs neighbor changes his Wifi password

I feel ya bro.

You either die using someone else's wifi, or you live long enough to get your own.

Or you could try to hack the router, just find its ip address type that into your browser and type the admin user name and password (most people never change them from default) and change or remove the wifi password.

I was actually told, using someone's else's wifi, (without their permission, I assume), is actually a federal crime.