I am not sure why this question keeps getting asked...
Look at your stock. Does the buffer tube go all the way to the rear of the CAR stock as most all fake CAR stocks do? If so, I hope you realize that you have a rifle length buffer tube.
Now, I guess you "CAN" unpin the CAR stock and I am betting that you may need to cut the original stock from the buffer tube as some use an epoxy to help keep it where it belongs (I could be wrong... so let me know if you do it).
From there, you will need to cut down the full length buffer tube and weld a new end to the shortened buffer tube. I also think you may need to drill new holes for the latch pin to engage to get your various positions. When you are done you may also need to replace a few other parts depending on any dammage you do getting things apart, cut down, rewelded, and then reassembled...
Personally, I would buy a new one for about $60 but if you really want to try to do it it should be possible!