If you have aircon, there's a tensioner and idler. No aircon - just the tensioner.
Replacement is pretty easy.
EDIT : Go in through the wheelarch access panel. Rotate the tensioner ( anti-clockwise ) with a deep offset ring spanner ( 15mm ), remove belt. Remove idler ( if you have one with the same spanner after prising off the dust cover. Remove the tensioner. Refit is reverse.
When you de tension the belt by turning the tensioner 90° ( approx ) anti-clockwise, you can slip the aux belt off the idler pulley. That'll allow you enough slack to work on the tensioner & idler. You won't then have to re-thread the belt over all the pulleys.
However, it's worth checking the belt for wear. As you're most of the way there it may be expedient to change the belt at the same time.
EDIT : Is your car CF3 engined? I know JTS cars had an aux belt guard over the exposed belt on it's way to the compressor. Not sure when that came in but it isn't a show stopper anyway.
Undo the small nut at the end of the guard that screws onto the aircon compressor mounting bolt. Then there's two allen bolts that go through the plastic timing belt cover into the engine. Fairly straightforward, but the cf3 may not have it anyway.