This is not a self-propelled model so I am assuming that you mean the engine itself is turning at too high of an RPM setting during cutting.
Typically the engines will run around 3200- 3600 RPM. If your engine is running faster than that you should stop using it until the issue can be addressed as any higher RPM than specified can cause serious safety concerns due to engine or even blade failure.
Since the unit starts and, I assume idles, normally the problem is most likely something with the governor assembly.
See items 81-83 in the images below.
It could also be an issue with the spring
or the linkage
that actuate the governor. Often the spring can come off or stretch.
Images such as the ones below from your
can provide a basic guide to help with the inspection but if no obvious problem is noted, your carburetor or governor may need adjustment. We recommend a professional make those adjustments to assure the mower can safely operate to its full potential and still meet all emissions standards.