I know this is an old post but I figure maybe my solution will be found by someone else doing a Google search and it can help them. Hopefully, you've gotten yours fixed in the 10+ months since you asked this.
Anyway, if you remove the top plastic shield from the mower (two screws hold it in place, plus a small cover near the air filter that needs to be removed) you will see a small spring that attaches to a tab at the front of the mower. If you bend that tab forward it will put more tension on the spring and increase the engine speed. You'll know if you are looking at the correct spring if you lightly press on the spring and the engine speed increases.
It is a good idea to monitor the adjustment with a tachometer. My owner's manual says the engine has a 3,100 max rpm. You don't want to set it above the specified rpm or you'll probably burn the motor up in a short time. I set mine at 3,060 rpm. It's hard to get it perfect but I figured slightly low is better than too high. Mine had been running at 2,800 rpm.
I bought a small engine tach on eBay for around $9. The wire wraps around the spark plug wire 4-5 times and then you can read the engine speed. It also keeps track of how many hours the engine has been running. I figure that will come in handy for oil change intervals so I permanently mounted it to the mower. It almost looks like it came that way from the factory. :-)
Here's the tach I bought:
Hour Meter Tachometer 2 & 4 Stroke Small Engine
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