Hi, and thank you for your question.
First off, please open up www.gmail.com on your computer.
When it automatically signs you in to the other account, click on the circle icon at the top-right corner of the email page and then click "Sign out".
It will then sign you out of the current account and take you back to the login page. There, click on the small "Sign in with a different account" link at the bottom.
Next, click "Add account" and then sign back in to your old Gmail account itself. This should do it.
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