iPad restore
iPad is logged in with iCloud username email that doesn’t exist anymore. Not ablea to log out sinceI don’t have that email address and password anymore.
How can I make it log out and log in with a new user?
You can’t log in with a new email address unless you can unlock the iPad from Activation lock. If this is your own iPad, (you are the original owner) take the device with the original proof of purchase to an Apple Store and ask them for help.