Already invited a user but need to change the email address? Or need to update an existing user account's email?
As a Super Admin (what are the different access rights?), you can head to 'Add / Edit Company',
Click 'View Users',
Click 'Edit Access' for the selected User Account,
Finally click 'Remove Access'.
Now that the existing account has been removed, you can navigate back to the Employee's Profile (Profiles - Employee Listing - Select and Edit a Profile), to input the new email address, then click 'Create and send invite'.