This video should help: https://www.diynetwork.com/how-to/skills-and-know-how/masonry-and-tiling/how-to-remove-and-replace-grout
Tedious job. You have to dig or grind it out without chipping the tile. Another option is to clean it and paint the grout. I used linen white oil based paint several years ago and it still looks good.
Hi Ed: Here is a site that you may find helpful: https://www.westlakehardware.com/videos/ask-the-experts/what-is-the-best-way-to-remove-grout-and-regrout-tile/