where is the tention to loosen the belts to change the altenator?

if you have a serpentine belt you will see the belt tention pully .

Without knowing what engine you have, it's hard to say. You should get a Haynes manual, which will tell you how to do this. In the older set-up, like the 302 from the '80s, the alternator was a tensioning point for the belt -- loosening and lowering the alternator released tension on the belt.

It really depends on year of vehicle and the engine type. ( some use serpentine belts and others use V-belts, and some use automatic tensioner , or manual tensioner) A good car manual , or shop manual would help you to do this.