You can change the shock oil yourself, but for recharging, take it to a shop. I work at an automotive dealer and we use nitrogen for checking for A/C leaks, so it's readily available to me, but kinda costly for someone to rent a tank, buy a guage and hose. Bleed off the reservoir before disassembling, then take apart, clean, and reassemble. Make sure to bleed all air from the shock before charging the bladder with nitrogen. Your best bet is to take it to a bike shop where they will do the whole procedure relatively cheap!