There are several types of fixed HK stocks, HK commercial surplus and US made versions. In the pic above is the stock buffer, remove the 2 screws. Most US made stocks do not utilize a 3rd screw in the rear, the buffer should slide out from here. If you have a HK made stock the buttplate needs to be removed to access the 3rd screw. Lock the buttplate in a padded vice like in the picture below, give the stock a quick pull and it will come off. Look down inside the buttstock and you will see the 3rd screw, use a long flat blade screw driver to remove.
A cobalt or carbide drill bit is necessary to take out the staked areas. If one is not available, a carbide 1/8th in Dremil bit, cut off wheel or hack saw will work. Just cut thru enough to remove the stacked area. Now place the flange tightly in a vice. Though the below pic is a A3 stock, place a screw driver and tap it with a hammer to break the end cap loose. It should unscrew easily from there. Push out the old buffer and spring and slide in the new buffer kit. Restack or loctite the end cap on. Reinstall the buffer and your done.