Little tabs poke through the carpet, on the back side there are little flat clips that hold it to the back side of the carpet. I think the only way to properly remove this is to remove the carpet, then you can get at the clips. You can probably pull the cover lose by hand, but if you don't remove the carpet the little clips will be under your carpet and could get lost. There is a possibility the clips are not there, most people don't even know the clips exist! So it's possible it was re-installed without them. If the carpet is shot, you can also cut around it and then remove the clips too.
I have also seen where some have the metal tabs folded over behind the carpet....I will look later to see if I still have pictures of the clips.