If you used Google Authentication and if you want to modify the email you're using to login, you should set up a password first. When signing up with Google, no password is created, but if you reset the password (in the Log in popup), you'll be able to define a password. This password will be the one you will need to enter when updating your credentials.
If you need to unauthorize your Google account, you need to do this in the Google App Dashboard. Revoking access to Google is not something that is done on the Bubble side.
You can delete your account in the Account page. Note that when you delete your account, we delete your personal data (email, Bubble usage, etc.), but we do not delete the apps you have created and their database. If you need to delete them as well you should delete the apps first.