I understood that serrated bread knives did not need to be sharpened, but one I’ve used for years does not seem to cut as well as it used to.
Is there a way to sharpen them, or just have to replace it?
If it’s a cheap knife, then it’s probably best to just get a new one. If it’s a good quality knife, the Spyderco Tri Angle sharpener will restore the edge. It’ll also make most every other cutting tool you have frighteningly sharp as well. Spend the extra dollars and get the diamond coated speed rods if you have a lot of knives that are pretty dull. The diamond rods greatly speed up the re-profiling of a blunt edge.
If it is a high quality one, you can usually send it back to the manufacturer to sharpen with little or no fee. If it is just a generic bread knife, you may be out of luck.
There are more simple household sharpeners that claim to work on serrated knives as well. I have one, and it does an ok job (it was a Dutch no-brand thing, so I can’t recommend it to you). It needed less expertise to use then the one Scumpup linked to, though. Just ask at a well stocked shop for household stuff. If they don’t sell it, you’d be better off with a new serrated knife. They are not that expensive, and keep sharp for a long time.