Usually the bottom is bent under and stapled. Then that piece is interlocked to the one above it. Then more staples on the bottom of the next piece and so on. Remove the corner and any other moldings first. When I replaced some on mine it was a 3 person job because the center of long sections wouldn't stay snapped. You just needed a hand to hold it in place so you could get the end snapped in.
2012 Chevy 3500HD Dually 4X4
Crew Cab long bed 6.0 gasser 4.10
2019 Open Range OF337RLS
Yamaha EF3000iSE
retired gadgetman