Hello
Do you mean you want to change the email address you use to log into your Microsoft account?
If so, Can you sign into your personal account via this link: https://account.live.com?
If so, go to Your Info > Sign in preference, Add an alias.
Add existing: this will allow you to add your gmail, as long as it is not already associated with another Microsoft account.
Once added, make this new alias your primary alias, then you can either keep or delete the outlook.com address. VERY IMPORTANT - if you do delete the outlook.com address, it is permanently and cannot be added back or recreated if you change your mind later.