The tool is just a long wrench really. You turn the tensioner counter clockwise to release the tension. Take care to just turn enough to release enough tension to remove the belt. If you turn too far you will release the tensioner and the spring inside it.

Put the new belt in place running over the tensioner last. Then again release the tension with the tool in the same manner.

You do not need the special tool any wrench will do that can reach the tensioner.