Go to
http://www.beesvillebeefarm.com/jeep.html and scroll down to picture 21 (has lots of yellow arrows and is just after the pic of lifting out the fan and shroud). The 2nd arrow from the left points to the serp belt tensioner. Use a 15mm socket on the nut in the center of the pulley, immediately below the arrow, and rotate the nut clockwise until you can remove the belt from the alternator only (leaving the belt in place otherwise makes reinstall easier), slowly let the tensioner return to place (e.g. do NOT let it slam back in place). Reverse to reinstall.
It is apparently possible but not easy to do this with the fan in place.