You signed into Plex via the Continue with Google button which will create a new account if there is not an existing one with that email.
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With your new account go to account page at https://app.plex.tv/desktop/#!/settings/account and change the username and email to something else. It does not matter what. Like add XYZ to the end of each. After saving the change, on that same page delete the account (link at bottom of account page). You have to do this because two accounts cannot have the same email and neither username nor email can be reused for 30 days after deleting an account.
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Log into your original account with your old email using the
Continue with Emailbutton. Then also go to. https://app.plex.tv/desktop/#!/settings/account and change the email to your new one. If you want to also link it with your Google credentials edit the the Linked Accounts section of your account page.