There is a special tool that you can use to remove the old grout. It is kind of like a scraper but has a point to it. You can then regrout with grout that has been untinted or tinted. It is not hard but can be time consuming. For a small area, you might find the project quite rewarding. My spouse regrouted the white tiles in a bathroom with brown grout and it looks quite nice. Go to a tile store and ask for the tool.