If it does not fit the crib
Cribs vary. Sometimes a panel goes back, and that is worth planning for.
Sending something back
The return runs through the retail partner you bought from, under the terms shown on their listing at the time. Their order pages will start it. We are not in that loop and cannot move it along, so the fastest route is always straight to them.
The measurement worth taking first
Most pieces that go back went back because of the crib, not the panel. Before you buy, look at the two short ends of the crib and note whether you can see through them, and check whether the frame is a full size one or a mini. Will it fit my crib turns those two answers into a short list, and it takes less time than a return.
If it fits but you are unsure about it
Read the fitting notes on the piece again before you take it out of the crib. A panel that sags is usually a panel whose ties have not been pulled snug, and that is fixable in a minute. If it still hangs loose against your frame, it is the wrong cut for that crib and it should go back rather than stay in.