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 favor and invest in knee pads.
Mix the grout according to the manufacturer's instructions, but mix it with an acrylic additive instead of water.
Let the grout cure overnight. The following day, use a large damp sponge or towel to buff the tiles clean.