Without seeing a photo of the plate it's difficult to advise. However it's probably mounted to a plate holder. If it is, use a flat screwdriver to pop open the holder, they usually hinge along the top and bottom edge, so half way up at either end is the best place to do it.
Once you've popped open the holder, then the plate may be stuck in position with adhesive pads, so use the screwdriver to prise the plate away from the holder. Depending upon how many pads they've used, you might have to pull hard or lever it free.
Re-fitting is the reverse procedure. You'll need some double sided adhesive tape or pads, available from DIY or car accessory shops.
Take care not to damage your paintwork with the screwdriver.
