How do you replace the oil in a 13 hp Little Giant Sump Pump?

I'm assuming you meant "1/3 HP" as Little Giant doesn't make a 13 HP model.

There is a small black circle plug on the outside of the pump body with 4 holes dimpled in its surface. That's the oil fill port. You can turn it carefully using the points of a set of needle-nose pliers or snap-ring pliers. BE CAREFUL not to damage the plug, you're going to need to reuse it. It's sealed with a small o-ring (generic o-ring size '012'). Drain the old oil from there, then fill slowly thru the same port. There is a small amount of airspace required inside the motor so don't fill the motor completely full of oil.

If the oil needs replacing, there's probably another problem with the pump that needs to be addressed as well.