As mentioned, there is no 'adjustment', the tensioner pulley is spring-loaded - didn't you notice it release when you removed the alternator?
Anyway, you'll need to lock the tensioner first. There's a 13mm bolt head in the centre of the tensioner pulley - get a spanner on it and turn it anti-clockwise to 'wind' the tensioner back against spring tension. There is a hole in the body of the tensioner, through which you must insert a suitable locking rod - an Allen key works well. Shine a torch in there before you start to see how it works.
Once the tensioner is locked, you can fit the belt. I would strongly suggest you get a new belt - they have a nasty habit of shredding , and getting into the timing belt area (I speak from bitter experience!).
Make absolutely sure the belt is routed correctly, and that the 5-peak belt sits on the 'outside' five grooves on the drive pulleys - there must
be an empty groove nearest the engine block.
Using that 13mm spanner on the tensioner pulley, take the strain off the locking rod, remove the rod, then slowly release the spring to tension the belt.
Then use a socket on the crankshaft pulley retaining bolt to rotate the crank a few times (clockwise looking at the pulley) - checking that the new belt runs true.
Sounds long-winded, but it isn't really - but get it wrong and nasty things can happen