If an apprentice's mentor has changed, you can make the switch on the site, as well.
Navigate to the Apprentices tab on the left hand side.
Select which apprentice you'd like to have their mentor changed.
Select the Supervisor tab.
The existing mentor will be marked with a red Mentor icon. Scroll down to the person who you'd like to select as mentor, and click on the blue Mentor icon.
You should now see the new mentor will now be marked with a red Mentor icon.
You are all set!