from my experience only and not necessarily the correct way.
first you need to clear everything off the front of the engine to both get to the tensioner and remove the alternator... intercooler pipes, air resonator box, glow plug relay control module and air flow box.
the small bracket below and left of the air box which supports the box to the engine block also needs to come off to get the alternator out.
to relieve the tension you need a long socket bar and a suitably sized torx socket.
on the tensioner is a small torx head casting, below and right of the centre bolt, which you place the socket on and turn anti clockwise to relieve the tension.
mine was broken off and a real pain doing the job.
look closely, there is a hole in the right hand side of the tensioner and body, about 2 o'clock, into which you push a drill bit etc to lock the tensioner when it's pulled back.
if you dont lock it you will have fun pulling it back to replace the belt.
to remove the tensioner completely do the same but once locked back undo the centre stud and lift the tensioner off
good luck with the change....it is a pain in the neck.
Last edited by Pappajohn; 12-26-2012 at 12:00 PM.