You take your belt off (I assume it's already off if the tensioner is broke). And you take a ratchet and remove the bolt holding the tensioner in place. It is reverse thread. My S-10 had a 305 from a caprice and it would cut the bracket often because the pully would come off the bearing.
The tension pulley should have a bolt that holds it on, allowing you to replace the unit.
replace 1996 nissan 200sx se 1.6 tension pulley
Disconnect the battery.remove the serpentine belt.Unbolt the pulley.
Loosen bolt in tension pulley, then loosen tension bolt. Replace belt. Tighten tension bolt. Last, tighten bolt through pulley.
There is no fixing it. It must be replaced.
The tension on a serpentine belt is automatically set by the Idler Tension Pulley. If you find it is not correctly setting the tension, the the spring may be broken.
Replace the tensioner assembly as one piece.
Call the parts store. They will be able to tell you if the pulley is available separately or not.
There should be a "square" cut into the pulley bracket that fits either a 1/2 inch or 3/8 inch ratchet or breaker bar to allow you to remove tension from this pulley allowing you to replace the serpentine belt.
with a 9/16 socket and ratchet
replace the belt with a goodyear bet
Release the tension using a wrench etc from the idler pulley.