cancel a flat

If you mean a accidental on a singe note that consists of a natural followed by a double sharp, you will need to add the double sharp accidental and use staff text for the natural (press F2 while entering the staff text to see symbols) and move it into position.

If you really mean Naturals before a key change then look under style->General... in the menu and select Accidentals from the list and change the option from only when changing to C Major/A Minor to one of the other options.