An employees location is based upon the location that they usually work.
To update the location:
Click on the employee you are updating the location for.
Go to Locations.
From here you are able to:
Select multiple locations where the employee works.
As long as the location is selected (with a tick in the box) it will appear as an option when the employee's pay is being processed.
Note: locations which have been configured as available to all employees, as well as the employee's primary location (set in their Pay Run Defaults page), cannot be unselected in the employee's locations page.