back up password

If your computer is a Mac and you checked to save the password to Keychain when you created it you can recover it by opening the keychain app.

The only other option is to guess it. Fortunately, you get unlimited guesses, and, if you are like most people, it is a password that you reused from somewhere else, most likely Apple related. To speed up your guessing click to change the password.

You don't.

If you can't remember the password, then you can't restore from that backup.

If you could "reset" it somehow, the encryption would be worthless.