HiCustomer I've attached the workshop procedure below. Basically you loosen the bolt at the front of the belt tensioner and then wind the tensioner down with the adjustment bolt. Once you install your new belt you wind the tensioner back to it's proper tension by aligning the gauge on the tensioner with the marks on the tensioner body. Don't go too tight or you'll shear the tensioner rubber. You have the first version tensioner. Let me know if you have any questions.
Remove And Install Poly V-Belt
The first picture in the document is the system you have it's just difficult to see on the car. I've attached another diagram of your tensioner and a couple of possible belt routings. Let me know if you need more help with it.