To kill off weeds in your grass look for a weed killer that is made to kill only weeds and not grass or other plants.
Get some weed killer that you put on your grass and it only kills off the weeds and not the grass.
You'd have to read the labels on the weed killers or ask in your local hardware or other gardening store if they sell the weed killer that doesn't kill grass.
I know it's made because I've seen it in stores before.