I'm not aware of any tools in Paint.net that will allow you to sharpen an image. Generally speaking, you don't get sharpening tools until you get into more sophisticated software such as Photoshop Elements or PaintShopPro. Perhaps one of our members would be willing to run it through their software for you (it would mean your providing them the high-res file). That said, blur is the one thing that can't really be fixed and while sharpening may improve that a bit, it could also enhance other, less desirable elements.