How do I take my younger brothers things off my email without deleting anything?

[Windows 11]

My younger brother has been using my personal email as his email for his user account, and i don't know how to change it off to his own email, whilst not deleting any of his stuff (it's connected to his OneDrive). The email is an @gmail account. Also if possible, I would also like to know how he can transfer/set up his OneDrive and other things connected on his own email?

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