How do I fix bathroom tile?

You can re-grout directly over top the existing grout. Since the present grout is already below the tiles you would only need to lightly sand the present grout w/a grout saw ( available at any home store ) to roughen it up and give the new grout something to stick to. Be sure to vacuum then wet clean the entire floor and allow it to completely dry before applying the new grout. Wipe the grout off the tiles as you go. Once the grouting job is complete wet wipe off any remaining dirty haze then dry the tiles w/a dry rag as best you can without disturbing the new grout. Allow the new grout to set & completely dry for a week before applying a layer of grout sealer. Good luck :)

https://www.youtube.com/watch?v=Wtb0_SafLzs

A more complex, but more attractive, solution would be to use a liquidy cement leveling compound over the entire floor which will cover the tile and grout. Then lay down new easy to clean flooring (i.e. click-lock laminate planks). Look up how to do this on the internet to see if it is something feasible for you. In the end, it will give you a considerablely better result that is permanent, fresh-looking, and if you don't choose grouting (please don't), there is no yearly grout cleaning and sealing.

From the picture, I would say your tile is Travertine, a natural stone. I would seal those tiles BEFORE I grouted. If you seal before, you won't stain the tiles & the excess grout will wipe off easier. Quick & easy w/ a roller for smooth surfaces. No need to trim the edges w/ a brush. I wouldvtry nog to get sny on that old grout, though. The new grout will bond best to unsealed grout. Tile sealer soaks in when the surcace is porous. Dries fast. The container will likely tell you how long to wait before wiping off any excess. Just a few minutes Using a large sponge for cleaning grout off tiles is much easier than a rag. Cleans a larger area & it kind of "floats" over the grout lines & won't mess it up as bad as a rag. Wiping diagonally across the grout lines rather than parallel to them helps to not remove the grout you want to stay, too. Look for the big sponges in the tile dept. at places like Home Depot. Bigger, thicker, tougher than the ones in cleaning supplies. And they don't crumble off & get in your grout. THAT is a problem you don't want to have! Clean well as you go. If that haze dries on the tile, it's much harder to remove, although you can buy a cleaner esp for that reason. I have a suggestion about color. If you choose grout in a color close to the tiles, you will not notice the width of those grout lines as much & probably make your bathroom seem larger also. If I was doing it, I would take the time to remove enough of the old grout in the holes in the tile so I could put new grout in them, too. If it won't come out in small holes, too time consuming, whatever, buy a bottle of the acrylic craft paint to match the grout & paint those holes. I did our master bathroom grout w/ Ceramacoat Acrylic Craft paint when there were stains I could not remove. A toddler & a permanent marker. We were in the house three more yrs & it still looked perfect. I didn't sand the grout. Just painted it. And an FYI. there are 2 types of grout. Sanded which must be used for deep and/or wide grout lines to help keep them from cracking. And sanded grout can be used everywhere else, too, if it is your preference. And Unsanded, which can be used for narrow &/or shallow grout lines ONLY. If your old grout is sanded, & has not been worn smooth over time, I don't think you will need to rough it up to get the new grout to stick to it, but do what you feel comfortable with. If it is loose or cracking, though, you really should remove as much of it as you can or your new grout will soon be loose, too.