Carbine receiver extension?
If so, pull all the way down on the butt stock telescoping lever (down, and not just cam'd to one side) and you can pull the buttstock off the back of the tube.
Now with the tube in hand, put a telescoping wrench on the castle nut, and back it off from against the back plate/back of receiver. The receiver back plate is going to back away from the receiver due to the rear take down spring tension, so grab the rear take down spring and detent (back plate is what holds them in into the back receiver channel for them).
Once the back plate has been backed away from of the receiver (bottom of the plate has a detent that mates into a recessed on the bottom back of the receiver), then you can spin the receiver extension back off the needed amount.
Note, when you assembly the tube, castle nut gets back way off with the back plate, you get the tube in place to index and hold the buffer retainer in place, load the take down detent point side into the channel first, spring behind it, then push the plate forward, and thread the castle nut up against the back plate, followed by final torquing the castle nut. To remove, just do in reverse, stating with backing off the castle nut.
Now having said this (and you have a generic knock off receiver extension/tube), the end of the tube has to hold down the buffer retaining pin, so you may have to shorten the end of the tube so you can get both, the tube to index correctly so the butt stock is straight up and down, and the end of the tube is just kissing the center post of the buffer retainer (tube over the larger main flat of the buffer tube pin top back side to hold it in it's channel.
13-6, D ( Page 200 if you are using acrobat reader)
If you have a fixed stock with full length receiver extension, then something is way out of spec if the top of the tube inside the receiver is blocking the upper from coming down. If this is the case,then I will need you to post some photo's.