I think he's meaning the rear idler hub?
You can do a lot to the road wheels just by jacking the arm up, but I'd be wary of attempting anything on the rear idler if the track is still resting on it.
Are you replacing -
the big bit with the holes that the two wheels bolt onto
the small bit that the goes inside this that's covered in oil
the arm and peg that it all fits onto (and the tensioner piston butts up against)
?
To release the tensioner piston pressure you need to undo the bolt (if you have one) or the nipple at the opposite end of the tensioner (where you nomally squirt the grease in to push it out) It's on a thread and you can undo it with a spanner. Take it out and (hopefully) the grease will squirt out (have a bowl handy).
If the piston is stuck, then you need to apply some pressure to it to push it back in, that could be a bit of wood to protect the end and a hammer. If it's stuck then b efore you reassemble everything, work it in and out a few times (add nipple and squirt grease in to push out, release nipple and hammer back in) to free it up. Some penetrating oil on the piston helps as well.