1. Remove buffer and spring from buffer tube. Remove castle nut with CAR wrench. Sometimes they are a little snug, get a hammer and tap the wrench. Might help to put your lower loosely in a vice, protected with a towel or whatever.
2. Back castle nut all the way off, then pull back car stock retainer plate. There is a spring there that goes into the lower that holds pressure on the detent for the rear takedown pin. Just be careful and leave it in there... or take it out (spring) for now.
3. Remove buffer tube from lower by unscrewing. Sometimes they are tight.... so the same applies as #1..... apply force.... it will bust loose. Once it breaks loose, go SLOWLY and hold the buffer retention ping/spring or it will launch across 3 counties. Unscrew all the way out.
4. Screw in new stock buffer tube. Push down on buffer retention pin/spring and screw tube over the top of it. Dont go too far... the tube should just barely cover the pin enough to hold the pin. If you go one revoltion too far, the upper wont close on the lower.
5. Push stock retainer plate against takedown spring and lower receiver, and hand tighten the castle nut. Then using CAR wrench, tighten up the castle nut, making sure stock is straight up and down. Some like to use a little blue loctite on this nut.... to each their own.
6. Insert new CAR buffer and spring.... assemble upper, and function test.