Here's some guidelines on editing users when using the Edit User API.
Edit Parameters - An existing user may be edited using user.php to update the user name and general information. Additional parameters can be edited using the Qualys user interface. When editing parameters using user.php, existing parameter values are replaced with newly specified ones. For example, if you edit an existing Scanner with the assigned asset group “New York” and you wish to add the asset group “Hong Kong”, then the edit request must include asset_groups=New+York,Hong+Kong
Clear Parameters - An edit user request can be used to clear (reset) parameters by assigning the empty string ““.