Brian Monetti, I restore Camaros
You should be able to release the tension from the tensioner with the 3/8 drive from a socket wrench and a little bit of muscle. The 3/8 drive will fit right into the square hole on the tensioner. You shouldn't have to take the fan or any other parts off, even though the space is pretty tight. You might need to use a long socket wrench or a length of pipe to make the lever arm of the wrench longer in order to get a bit more torque out of it and prevent your knuckles from bashing. Good luck!