Turn the idle screw out? Figure out why it's revving too high. Is the choke partially on? On some motors putting the choke on also opens the throttle, check if yours is like this. Did the problem just start out of the blue, or after some work was done on it?
If the carb was recently cleaned, then either something is wrong with the throttle linkage (holding the throttle butterfly partway open) or if someone actually messed with the screws on the throttle butterfly they might not have the butterfly centered properly so it can close all the way.
