Take a look at some drawings that team member Magoo6541 posted link, look for the lower half and also this
Lower Receiver Assembly - GUIDES link.
Both A2 and telescoping stocks are shown there.
Remove your buffer and action spring by pushing down the buffer detent pin at the rear of the receiver.
Remove the upper stock screw, it will take a good force to take it out because of red Loctite on the screw.
Slide your stock back and remove it also remove the thick spacer.
Turn the buffer tube counterclockwise to remove it from the lower receiver - BE CARFUL WITH THE BUFFER RETAINING DETENT PIN, cover it with plastic or shop towel to prevent launching it.
You will need a RRA carbine extension tube or any commercial type tube. Mil-spec size tube like Vltor, LMT, Stag Arms, Colt, etc. will not fit RRA or commercial type buttstocks.
At the bottom of the instructions that I provided is the telescoping stock procedure.
There is another thread that shows how to install the CAR stock -
Assemble Your Own LOWER, UPPER, FREE FLOAT, TRIGGER, GAS BLOCK - Step by Step Instructions! .
I have both A2 rifle and carbine ARs which I built myself.
Good luck.