An above ground pool liner is the vinyl shell that holds the water, and it is the part that wears out first. Sun, chemistry and age fade it, stretch it and eventually tear it. Replacing a liner means draining the pool, taking out the old vinyl, checking the wall and the base underneath, and setting a new liner so it fits without folds.
The installers on this page say they replace or install pool liners. Before you book one, ask whether the price covers draining and refilling, whether they check the cove and the base for damage, and which liner pattern and thickness they carry. Ask, too, whether the liner comes with a warranty and whether the labor does.