Can't be done in Lightroom. Photoshop has a tool for this.
How blurry? If it is minor you can get quite a ways in Lightroom by using the detail tools. There are many tutorials on how to sharpen in Lightroom online. Most important thing is to zoom 1:1 before you touch the detail sliders. If it is blurriness caused by camera movement, you should check out the Sharpen->shake reduction filter in Photoshop as you can't fix that in Lightroom.