How do I replace the Alternator belt?

First, yes, if you are replacing one belt you should replace them all as they all have the same amount of wear unless they were already replaced. Plus, you have to remove the outer belts to get to it, so it's easy to replace them. So, you first release the tension on the outer belt (power steering) using one of the methods shown in image 2 or 3. They used different methods depending on the parts available so it will either have the tensioner bolt or the lever version. You release them and remove the belt. Next is the AC belt, it comes loose by loosening the bolt in the middle of the idler and using the tensioner bolt similar to the PS belt. Now you are down to the alternator belt. For it you loosen the bolts shown in the last pic and release the belt. Next you install the alternator new belt and tension it the reverse of how it came off, you are supposed to use a tension gauge for all three new belts but your finger pushing at about 20 pounds of force and a ruler works as well. For all three belts the tension is about the same, tighten the belt until you can measure between a quarter inch to three eighths of an inch (6-8mm) of deflection on the new belt when you push on the longest section of belt. Tighten the various bolts and such down to the numbers shown. On the AC system the last image shows the mounting bolt, you tighten the bolt you loosened to adjust it the same amount.

I usually reach up from underneath if possible that gets you under the hose that goes over it.