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The goal is to release the tension applied by the spring-loaded tensioner.
I use extended sockets placed on the bolts of the tensioner pulley and tensioner base, then use a crow-bar or the like to apply leverage to lift the pulley. Be sure to keep the lever as close to the pulley and base as possible, so the sockets won't slip off the bolts.
Take things slow and easy, keep fingers and other mashable body parts out of the way, and only release enough tension to slip the belt off the alternator pulley.
Before removing the belt, make sure you have the belt path committed to memory, sketched out on paper, or have located the routing diagram - typically stuck on the radiator structure support, but it may be long gone given the age of the truck.
Route the new belt around all the pulleys, with the exception of the tensioner. Apply just enough leverage to lift the pulley high enough to slip the belt under. Again, watch out for your fingers, should the lever slip.
Should be good-to-go.