Here in the UK loosing the outer skin of the silencer is not an MOT (annual safety check) failure. Just cut it off neatly. No need to replace it if it's not blowing. Unless you want to of course.
OE systems are often a single piece.
If replacing just one part, line it up and mark the cut point carefully. Remember the new piece has to overlap the old, so don't cut too much off.
But, as said, the outer layers peel off without affecting performance or integrity, so will last a long time like that. I think there are three layers, so only needs changing when the last, inner layer fails.