You've got about 6 screws underneath holding it to the car plus i think a few hidden in the wheel arch liner maybe aswell as all the clips along the top edge of the valance. Many people say its easier to remove the whole bumper but can be done with the top half still on the car
Dave
^^This.
Torx (20/25?) on the underside and a few in the rear arch liners, two bigger torx (30?) on the underside (these usually go brittle and snap so take care).
When all torx are removed, start at one end, finding a balance between being firm but gentle and pry the valance off clip by clip. Take care not to pull too much as it might flex it and cause white marks/cracks.
Do this all the way along. Refitting is the reverse but I'll tell you now, if you have big fat hands or arms you might struggle to get the clips near the numberplate in (when you push they flex as nothing behind to resist). I manage to get my hand up behind it to add resistance when pushing in, but you may need to improvise/or remove the whole bumper if this will be an issue.