I just used them until they got dull, then bought a new one (I actually have 3 blades total).  I saw a YouTube video with blades in similar shape, and they used an angle grinder to sharpen them.  Critical part I learned is to balance them, which is also appears easy.  I bought an angle grinder from Harbor Freight tools yesterday, so I'm going to give it a try.  The underside of the mower looks like it good use a good cleaning too.  Another thing to add to my to-do list, (sigh).