How to repair grout in shower floor ?

You would need to remove the grout either with a gout saw, screwdriver, razor knife. Then you can regrout.

You will need to chip or grind out existing grout. Then clean the gap and add new grout to match existing (unless you want a different look). There is also grout that comes in a caulking gun tube. This is the easiest way to grout tile. I use it all the time when grouting backsplash tile in Habitat houses. Be sure you let the grout dry completely and seal it when dry. Grout is porous and you don't want it allowing water into the subfloor or allowing a place for mold to start.