once you unscrew your numper you'll see the four push pin rivets if its been fixed on properly
Google for 'Push Pin Rivet' and go to images - once you've got an idea of what your dealing with it'll be much easier - dont take a drill to it - thats overkill for a few tiny plastic push rivets!
you'll notice theres four 'notches' around the circle
use something thin, rigid and flat to get into those notches and pry out the centre flat 'pin', it should eventually come out totally (that centre pin goes into the other round bit which goes through the plinth and bumper and 'spreads' them to hold them in)
then do the same to all four corners
then very carefully pry out the centre pieces too
then it'll just pop off
be careful not let the numper plate plinth swing down on one pin as you may scratch your bumper...ask me how i know!