Currently, there is no option to update/edit an admin's email address in Partner Center. The best option is to create a new admin profile using the new desired email address and then delete the old admin profile if it's no longer needed.
Navigate to the Administration tab > My Team to create/delete an admin profile.
To delete a team member, click the three dots next to the team member's name, and select Delete Member.
To add a new team member, select Invite Team Member from the top right-hand corner of the screen.