1992 Buick Century tension pulley

If it's just the pulley, it's easy; you just remove the nut at the center and pop the new one on. The tensioner, however, which I what I suspect you mean, is a good deal more involved, since it's integral to a large bracket that also supports the power steering pump and half of the alternator. After removing the belt and those two components, you'll see main bolts at the top and bottom of the bracket/tensioner, which is a kind of vertically elongated diamond shape. The trick for the power steering pump is three bolts accessible through the holes in the pulley.