Most likely, you will need access from both top and bottom. Really depends on how much access you have from the top. If full access, you can do all of the serpentine belt from above.
BUT, with the Caterpillar 3126, you also have a separate V belt that only turns the water pump. If replacing belts, be sure to do that one as well-- from any Caterpillar dealer. It is only serviced from below and has a standard adjustable tensioner, not automatic tensioner like the serpentine belt. This belt and its tension is an often overlooked item.
