As mentioned, that blade doesn't look too bad. I do keep mine sharpened, and have at least two sets for each deck. That way there's always a sharp set ready to install if I'm in a hurry.
My favorite method is with a flap disc on an angle grinder. Of course, the wheel rotation should be so the abrasive travels toward the sharp edge. Otherwise the sharp edge can make short work of the disc. The flap disc makes it easy to get a nice smooth surface without the gouges a hard wheel can produce. A final very light touch to remove burrs and check for balance. I use one of those cheap stepped cone tools.