Replacing the belt is quite simple: Insert a 1/2" breaker bar or wratchet into the square hole in the tensioner and release the tension on the belt, which is already done in your case, but you still have to lift the tensioner to slip the new belt in place.
When you lost your belt, I hope you learned the valuable lesson of keeping a spare belt with you, especially in the case of a serpentine arrangement when you lose everything if you lose the belt.
_ken