Coffee burrs can be sharpened, but it's important to know how to do it properly. Otherwise, you could end up damaging your coffee grinder.
The first step is to remove the coffee beans from the grinder. Then, take a look at the blades. If they're not too dull, you can try sharpening them with a honing rod or a sharpening stone. Just be careful not to overdo it - you don't want to create any nicks or uneven edges on the blades.
If your coffee grinder's blades are too dull for honing or sharpening, then you'll need to replace them. You can buy replacement blades online or at most kitchen stores.