To sharpen a pencil sharpener blade on a manual sharpener, carefully unscrew the top screw(s), turn it over to allow the blade(s) to drop out, and use a sharpening stone to sharpen them before reassembling. To help maintain sharp blades, be sure to empty your sharpener of pencil shavings and use a dry spare toothbrush to keep blades clean and sharp. You can also try cleaning the blades with rubbing alcohol which may do the trick by removing any gunk that is clogging them up.