How do you repair/replace floor grout? ...
Answers: You can regrout by digging it out, but it is a major amount of work. You may want to skim the top of the grout next to a new coat, or bleach it.
With a grout saw and profusely of elbow grease...be sure to remove all existing grout earlier re-applying; If you put new grout over the out-of-date it will crack and break..............