The tensioner is an automatic (i.e. spring loaded) type. You need to get a spanner or socket on the sprung tensioner bolt and rotate it to release the tension, then thread the new belt back around the various pulleys and the fixed tensioner (idler). Not sure which way you turn the tensioner to release the tension - models with aircon have a different pulley arrangement than those without. It should be obvious which way you turn the tensioner to relax the tension once you get a good look at the area - not easy when looking down the narrow gap from the top.
Oh, and mind your fingers!
It's best to gain access via the wheelarch - remove the plastic liner and you'll get better access and a better view of all those pulleys. Need to replace my sprung tensioner soon myself - haven't actually done the job but got advice from someone who's had their Multi to bits (thanks JTDMan!).
Hope this helps.
