The plastic will not come off by itself, the whole bumper assembly must be removed.
Its 3x or 4x 13mm bolts on each side of the bumper stanchions, and i think 2x 10mm bolts on each side which fasten the plastic to the fender. Start soaking the side bolts in penetrant now, they bolt into clips on the inner plastics, and any bit of resistance or rust will tear them out of the plastic.
Once you get the whole assembly off, you'll see the 3 studded strips along the top that hold the bumper cover on. Those could probably use soaking too, if they're rusty.