Admins can update a User's email address by going to Admin > Users > click on the name of the User to view more details. From the User's Details Page, click the Edit button in the top right corner to update the email address. Keep in mind that updating the email address also updates the User's login credentials, so they'll need to use the new email address to log in to the portal. 
User Profile
Can Users Update Their Own Email Addresses?
If Single Sign-on (SSO) is not enabled for your portal, you can give Users the ability to update their own email addresses by making the Email field in the User profile section visible. Go to Settings > User & Organization Fields > User Field Requirement & Visibility.
User Field Requirement & Visibility Settings Page
Users can then update their email addresses from their User Profile settings. They can access their Profile settings by clicking the profile icon/picture in the top right corner of the portal > User Profile > Edit.
What Happens When You Update a User’s Email Address?
Updating a User's email address also updates the User's login credentials. Moving forward, the User will need to use the new email address to log into the portal. Email notifications will also go to the User's new email address.