Thanks for posting here in the Community, jennifer9.
I'd be glad to lend a hand with changing your employee's email address.
The master admin should go to https://accounts.intuit.com/index.html and sign in. You will then see an option to edit their email address when signing in to QuickBooks Online.
The following is a screenshot to serve as a visual reference:
This article provides further information about updating user's information: About managing another user’s information.
The steps above should help you update the employee's email address. If you have any questions about this or anything else, please let me know. Have a good one.