Place the hedge trimmers on a flat, hard surface. Make sure it's unplugged or that the spark plugs are removed if you have a gas run hedge trimmer.
Follow the manufacturer's instructions to remove the blades from the hedge trimmer. You will most likely remove the housing and then loosen and remove a couple of bolts. Turn the bolts counterclockwise, using a pair of pliers.
Tighten the blades in a vice and file the edges, going with the factory grain, until sharp. Avoid filing for too long in any one spot. This will cause the blade to get grooves. You will know the blade is sharp when it no longer offers much resistance and can cut through a piece of paper with ease.