You must drain the old fuel mix before doing anything, it can make a world of difference. Old fuel leaves nasty deposits that mess up the carb operation. You'd be well served to clean out the entire fuel system with carb cleaner and replacing all filters. Replace the spark plug while you're at it.
With hopefully a clean system and new plug, and certainly fresh fuel mix and clean filters, try it again, it may work fine. If not, you may just need a minor carb adjustment. You typically need to locate the slow idle mixture screw. I'm not familiar with this engine, so the procedure may vary. Check the owner's manual, they should have a section on idle adjustment. One typically adjusts the idle screw so the engine runs fast enough to not stall, but not so fast the head turns.