>>2932140 (OP)A file. It is almost ALWAYS the answer to 'how to sharpen'. People are retarded and quick to go to abrasives, often rotary tools. It's a waste of money, produces ugly swarf instead of filings, when not looked after the geometry is random and in the case of powered abrasives you risk drawing the temper of your blade.
So file it.
If you want something to be razor sharp follow up with a slip stone. But a hedge cutter isn't really the type of thing you need or want that sharp. Save yourself the time.