If the box is metal, you don't need to. The switch will be grounded via the screws (assuming the box is grounded).
If you feel the need to ground it, there will be a 10/32 tapped hole in the back of the (metal) box for a ground screw.
If the box is plastic you will need to add a pigtail to the existing ground wires.
If there are no ground wires, you are probably not going to have a ground available anyway. Move on.
If you feel the need to ground it, there will be a 10/32 tapped hole in the back of the (metal) box for a ground screw.
If the box is plastic you will need to add a pigtail to the existing ground wires.
If there are no ground wires, you are probably not going to have a ground available anyway. Move on.
