IDK where you live or what kind of grass you are cultivating which are the variables here.

If there are just a few spots you can pick out by eye you can apply glyphosate (Roundup) at label dilution either as a spray (it will kill EVERYTHING it touches) or with a medicine dropper (if you need to be more selective).

There are some high dollar (few hundred dollars/oz) selectives you can use on warm season grass like bermuda and zoysia, but they are probably not worth the trouble.

Next year toward the end of summer you can also apply preemergent and either prevent or diminish weeds you might be seeing emerge now like poa annua, henbit, creeping charlie, etc.