In this video I show MY method of sharpening a serrated knives. There are several others and you can make them as simple or as hard as you like.
I didn't invent this video, I just learned the technique from watching other sources and then putting them together. I have tried several methods but this is my favourite.
If you don't have the gear I am using, try being inventive and make it simple, all you need to do is remove that fatigued metal from the serrated side and ensure the flat side is clean of any debris. A Steel may do the trick, be gentle and use just enough pressure to get the job done.
As I said, I didn't create this method and I have fiddled with it over the years. I have used micro abrasive wrapped around dowels to do this. You can try that as well if you like.