A gear lube change isn't likely at all to fix that transmission, being stuck in 3rd gear isn't a minor issue fresh gear lube is likely to fix.
But if you want to try anyway, the drain is at the bottom front passenger side, the fill is half-way up on the driver's side.
Our TJ transmissions are a bit fussy about the lubricants they run on so you can't just use a generic "Manual Transmission Lubricant" like Autozone or NAPA sells. Suitable lubes are a GL-4 75W-90 gear lubricant (VERY hard to find, do NOT use a GL-5 75W-90), Redline MT-90, or a good synthetic 10W-30 motor oil.
Don't worry about the seemingly huge viscosity difference between the 75W-90 gear lube and 10W-30 motor oil. Gear lubes and motor oils are measured on two different viscosity scales & those two lube have very similar viscosities. The manufacturer of your AX-15 approved the use of a synthetic 10W-30 motor oil several years ago since the other lubes I mentioned that were on their original list of approved lubes are very hard to find locally.