Simply write a letter include your filing receipt number to CIS inform them that you withdraw the application. It may be the case that CIS still approves the application. If this happens, you have to apply for change back from F-1 to F-2. Otherwise you will lost your legal status if you do not intend to go to school as a full-time student in order to keep F-1 status.
If CIS questions your first application from F-2 to F-1, by then you can simple reply them to withdraw the application.