If you’re trying to change an agent's email address in the admin interface and you get the error: "Email address already exists". This is because the Email Address is already associated with a User on the helpdesk.

You can use the Global Search app to locate the User profile from the email address.

If this is the email address that is associated with the individual who will be the new Deskpro Agent, you can convert the User into an Agent account, change the User’s email address, or delete the User profile and then try and create the Agent Account again.