Remove the air cleaner from the top of the engine using a socket and ratchet. The air cleaner is on top of the engine, off to the side. Some mowers may have wing nuts instead of hex nuts.
Look for a vertical arm with springs attached to it. This is the governor.
Bend the arm slightly the left using the needle-nose pliers. Think in millimeters rather than inches when you make the adjustment. A tiny movement can mean a significant speed change when adjusting the governor.
Replace the air cleaner on the engine and tighten the retaining nuts. Start the engine. If you adjusted the governor too much, repeat the process, except bend the vertical arm to the right a little. You might notice the difference in the engine speed when you start it. If it sounds like it is running too fast, you need to adjust it back.