This thread will walk you through the procedure to replace the entire tensioner. However if you are just replacing the belt most of the procedures remain the same. It's a very straightforward job.
DIY - 1987 300D serpentine belt tensioner replacement
With that being said it is possible to replace the belt without taking anything out. It's a bit of a pain but you can pass the belt over the cooling fan and onto the pulleys. I can't explain how it's done but i've done it before. Now that I have the pulley holder i'd much rather just take the fan off.