Tile grout sustains a lot of wear and tear over time, especially in high-traffic areas such as the bathroom, kitchen and mudroom. Replacing old grout is not that complicated. All that is required is taking out the old grout and laying the new grout. You spend a fair amount of time on your hands and knees, however, so do yourself a favour and invest in knee pads.
Use carbide-tipped scraping tools to remove old grout. Scrape away the old grout, taking extra care not to scratch the tile.
Vacuum the old grout and go over any missed spots.
Mix equal parts distilled white vinegar and water in a bucket. Wipe down the floor with a large sponge and the vinegar solution.
Let the floor dry, then rinse it with clean warm water.
Mix the grout according to the manufacturer's instructions, but mix it with an acrylic additive instead of water.
Apply the grout with a hard rubber-bottomed float. Hold the float at a 45-degree angle so you can remove grout from the tile while applying it to the grout lines.
Wipe the tiles after about 15 to 20 minutes with a large wet sponge. The tiles will be hazy.
Let the grout cure overnight. The following day, use a large damp sponge or towel to buff the tiles clean.
A V-tool has a triangular-shaped head and gets between the tiles really well, but keep a variety of tools --- two or three --- on hand to work with grout in different stages of deterioration. According to Tom Silva of "This Old House," acrylic additive creates a good bond, provides some flexibility and creates a good seal.