Use a wrench to remove the two bolts that hold the old footboard's sides in place. Set the bolts, washers and nuts in a safe place so you do not lose them.
Lift the old footboard away from the bed. Position the new footboard at the bed's end. Line up the brackets' holes with the holes on either side of the footboard's legs.
Push two bolts through the lined-up holes on both of the footboard's legs. Slide the washers over the bolts. Screw on the nuts and tighten them to hold the footboard in place.