You loosen and take off, the belt. Then you take out the bolts that hold the alternator in place, and then put on the new alternator and replace the bolts and belt.
Picture say's a thousand words, here is a link for you on how to replace the alternator on your Jimmy. You alternator in your jimmy is a CS-130D and you can upgrade for the CS-144 alternators which is used on the emergency viehicals to run the extra electrical equipments (light's and Sirens ect). I am running the CS-144 alternator on my 1996 Jimmy with a double rectifier and upgraded bearing and has been in my Jimmy now since 2001 when I last upgraded the alternator. Over(200,000 miles on alternator)
The larger CS-144 mounting bolt pattern is the same as your factory standard CS-130 or CS-130D alternators. Good luck and this job should take you less then 30 min from start to finish to do and if your good with tools, you time will be less.