To add to what PX said.
Here are a few tips for changing out your stock.
Take out the buffer and spring BEFORE you start unscrewing the buffer tube.
After lifting your upper reciever up off of the lower, you will see the threaded end of the buffer tube right there. Inside of the tube you'll see the face of your buffer. Most buffers dont have a perfectly round face, and you'll notice flat spots on it. Simply turn the buffer till one of the flat spots is directly over the retention pin which is at the 6 o'clock position. This pin is spring loaded and is usually the one to go shooting across the room to get lost. Using a pencil (or something similar) press down the retention pin while at the same time pushing the buffer back slightly. As soon as you have the pin deperssed, ease pressure off of the buffer and slowly allow it to come forward. You should be able to wiggle the buffer and spring out of the tube at this point.
If you have a hard time rotating the buffer in the tube to align the flat spot to the pin, simply use a rag to help grip the bolt face and turn it. If you have sweaty hands or grease on the buffer they can be hard to twist.
Loosen the Castle nut. DO NOT start twisting the stock assy to unscrew it vigorously!!!! There is a spring that protrudes through the lower reciever on the right side at about 2 o'clock that puts pressure on the tensioner for the rear take down pin. This spring is loose and can easily be twisted to oblivion as you unscrew the buffer tube. The plate in front of the castle nut has a tab on it that prevents it from spinning free. The tab fits in a groove in the buffer tube, so when you twist the tube, the plate will twist right along with it. The spring in question isn't under hardly any pressure and won't come shooting out or anything, but it does stick past the hole a little and it's this loose end that can get mangled from rubbing against that plate as you unscrew the tube, so as soon as there is enough room to get to the spring, pull it out.
I know this may sound complicated, but it truly is easy once you do it.