I did it myself, i just found the Emissions Maintenance Timer in my Jeep Wrangler 1991, Its a clear brownish box under the dashboard with small plastics gears on the inside "like a clock" its factory set it for 2750 hrs. you just need to look at that box in the back and remove the sticker to gain access to a small opening where is the activated switch
, and with a small screwdriver move that switch from the top of that small gear to the border of the gear where its a guide for the switch. disconect the + terminal from the battery and touch the negative terminal with it the reconect the + terminal and thats it.