Are you talking about the rear crossmember? There's the frame rails coming back each side then the crossmember is flat steel with some holes in it. It sits flush with the back of the tub. Any bumper/bumperette will bolt onto this. So if you need to replace this, you have some frame repair ahead of you.
Otherwise it should be just unbolt what bumper is there and bolt the new one on.
Otherwise it should be just unbolt what bumper is there and bolt the new one on.