Google problem

[whitjs]

A while back, a friend was on my computer and somehow got his email and phone number  on my google account.
Now it appears this info shows up and he is the owner of the google account on my computer.
Everything I sync and do with devices comes up with his email and phone number.

How do I get this off my computer so I can get the account under my name? What will I loose and how do I get it back/

Yes, I'm not really good with the computer but can manage.
Thanks in advance for any help.

[TheWaffle]

Is this what you are looking for?

https://support.google.com/accounts/answer/3463280?hl=en&co=GENIE.Platform%3DDesktop#:~:text=You%20can%20add%2C%20update%2C%20or,actions%20like%20changing%20your%20password.

[whitjs]

Partly. I need to get rid of the gmail on the account and the phone # and start fresh with a new account.
I also wondering what I wlll loose on my desk top when I do this.
Thanks for helping.