Open an encrypted backup
The backup that arrives by email is encrypted with your account password — the same one you sign in with. Choose the file, type the password, and get an ordinary backup file back.
This happens entirely in your browser. The file is never uploaded, and the password is never sent anywhere. You do not need to be signed in, which matters if the reason you are here is that you cannot sign in.
If you have changed your password
Each backup is sealed against the password that was current when it was sent, so older files need the older password. Backups sent from now on will use your current one.