It’s hard to tell from the picture, but the solution is the same regardless of the type of unwanted grass you have. You have several options. You can selectively remove the unwanted clumps and replace them with healthy sod, which in your case might be a big task. Or you can carefully spray the unwanted grass with an herbicide (see the link below before attempting), being cautious not to get overspray on desirable grass, remove it when it’s dead and replant with sod or allow your lawn to fill in the bare areas. The other option would be to consult with a lawn care company in your area. They are often able to apply herbicides that remove unwanted grass types while at the same time applying fertilizer to the desired lawn to improve it’s health so weeds and unwanted grasses can’t invade. The key to keeping lawns weed free, which includes weedy grasses, is to adopt a lawn maintenance program. Healthy grass is much less likely to have unwelcome weeds and grasses.