Right, if you have clear access to the tensioner this shouldn't be too hard. You need to remove the rubber bung at the back of the tensioner, then insert a screwdriver to hold the screw inside in position. Don't turn the screw left or right (you may need a little helper to hold this for you).
Next undo the two allen screws. The tensioner unit will slowly push it's way out, but remember not to let the screwdriver turn or the screw turn.
Once the unit is out of the bike you can let the screwdriver turn, but make sure it stays on the screw head. This will tell you if you have any more travel in the unit, and/or if the spring inside is broken.
If all is well the tensioner will want to push outwards and spin the screwdriver with a fair bit of force.
To refit the unit you need to screw the "pusher" all the way back into the unit (you'll see what I'm on about when you remove it). Now you need to keep the tension on the spring with your screwdriver as you line the unit back up with the engine, and bolt it back in position. Now let the screwdriver turn and the tensioner unwind
If it doesnt' start turning the screwdriver by itself give it a little hand as you may have backed the screw out enough to get it a little tight.
Again you'll understand what I mean when you have a go at it
Hope this helps and I'm sorry if it's a bit vague.
Bullet5.
