In the Services Control Panel, right-click the qubesupervisor service and select Properties from the menu.
Click the LogOn tab and then
click the radio button to set "This Account" with the proper login and password.
Remember, the supervisor will read from and write to the database and log files. The user under which it runs must have read/write access where appropriate. PipelineFX advises that you do not change the user under which either the worker or supervisor service runs unless absolutely necessary.