Serepentine bels do not have tension adjustments, there is a tension pully That is preloaded with a coil spring in it. If the belt is loose, the pully spring may have lost it tension. Replace the pully assembley.
you do not adjust a serpentine belt. the adjustment is done automatically by the tension pulley if belt is loose, purchase a new belt . also check that all pulleys are in good shape before installing new belt