Inspecting the Damage
Before removing old grout ensure that the underlying tile is not damaged by tapping carefully on the tiles in several places. All the tiles would have to be replaced if you hear a hollow thud.
A solution of 1-part vinegar to 1-part water should be used to clean the area where the damaged grout is located.
Removing Old or Damaged Grout
Grout saws should be used to loosen and remove old grout. A small lever-type can opener with a pointed end can also be used to decrease the chances of damage to the tile.
Removing Grout from Corners
A utility knife or screwdriver should be used in removing grout from corners around bathtubs and kitchen tubs.
Grout getter is a special tool having a triangle-shaped carbide or diamond blade for removing grout. The point of the blade is placed on the grout and the tool is moved forwards.