First you must release the tension off the belt. Look in your under hood area and there should be a belt routing diagram. Once you are sure of the routing, then you can rotate the Tensioner so it releases the belt tension. There is typically a hex area where yo can apply torque and then you slip off the belt.
The tensioner is held in by bolts, so loosen them and remove it. I don't recommend just doing the pulley, because the spring in the tensioner is worn out about the the same time the pulley fails.
Then reverse the process, that is re-install the belt.