RealOEM.com * BMW E83 X3 3.0i M wheel arch cover/ sill
items 6 and 9 are plastic rivets , assuming your car has these , you can carefully pull the centre pin out until it stops which will allow the expanded outer part of the rivet to fold thus releasing it`s grip .
you could drill out the centre part if it gives you bother . replacements are cheap from a dealer , maybe you should buy some to give you an understanding of how they work .