loosen the alternator, by unscrewing the bolt where it connects to the bracket on the motor, then under the alternator, there is a belt tensioner and you have to move that screw down to loosen it and then tighten it up to make the belt tight.
if you see those thin bars by your hand in the picture there should
be a bolt that you loosen to be able to move the alternator.
right under on the alt.
try to make sure you put the bolt in the same spot, for
the right tension on the belt. if it still is hard to move you can
also loosen the bolt that holds the alt. to the motor, right above
your hand by the alt.
hope that helps