There are two types of these...
A) The type with a small concentric circle in the center of the fastener.
For this type you can use most any small object, such as a small phillips head screwdriver or small hex key. Just push the center straight in. Once the center section is depressed, the fastener can be easily removed.
To reinstall, push the center section in the opposite direction back through the fastener until it is protruding from the top side. Replace the fastener in the bodywork and use your finger to push the center section in until it is flush with the outer rim of the fastener. The center piece should snap back into place.
B) The type with a larger circular head with two small horizontal slots which are located on opposing sides of the head.
For this type, just insert a flat blade screwdriver in on of the small horizontal slots and gently twist the screwdriver until the head of the fastener pops out.
To reinstall, leave the head of the fastener in the unfastened position, place the fastener back into the bodywork, gently push the head of the fastener into the center portion until it snaps into place and is nearly flush with the surface.
If you have to force either of these to reinstall them, that means that the fastener is not inserted all the way into both pieces of bodywork or the holes in the bodywork are not exactly aligned. Both types should easily snap into place using only finger strength.