It's a matter of bending or cutting off the current piece of metal that acts like a spring, and then replacing it with a spring from a pen. Try searching through this forum or you can also check out the BT Warrior forums for specific details with pictures.
If it's Gen1 the selector switch positions will be painted on, if it's Gen2 it's stamped into the plastic. Also the Gen1 stock will slide all the way out without stopping, the Gen2 stops at the end.