Start the snow thrower and allow the engine to warm up for a few minutes if necessary. Turn the engine off. The engine should be warm but not hot.
Look for the oil drain plug near the lower front section of the engine. Clean around the drain plug with a rag to remove dirt and grease.
Place a drain pan under the oil plug. Remove the oil plug with a socket wrench and allow the oil to drain into the pan below. Dispose of the old oil according to local regulations.
Install the oil drain plug and tighten with a socket wrench. Find the oil fill cap on top of the engine and remove it.
Fill the oil reservoir with motor oil. Check the oil level during the filling process to make sure you don't add too much. Replace the oil fill cap when the reservoir is full.