How to replace floor grout!?

Remove any loose grout and Bleach out the remaining grout making a paste of baking soda and peroxide and letting it sit on the grout overnight if possible. Then rinse it thoroughly. Many times you can grout to freshen it up right over the old grout. Otherwise you will need a tool such as a dremel a steady hand and safety glasses.

After you grout save your self from this experience by sealing the grout with a proper sealer and an artist brush.

You stand a good chance of chipping the tiles that way. If the tiles have a good glazing on them, you could use pool acid to eat the old grout out. Test a hidden area first to be sure the acid does not damage the tiles. Be VERY careful with this tho. The fumes will be EXTREMELY hazardous. Only do this in a very well ventilated area.

if it is the color you are not happy with home depot has a grout paint that works good. they have all kinds of colors. If not chip away

Tools

it would work,

but the work you going true, you will give up 100%