Reseeding old K31 lawn
We viewed your photos and they were not in focus. The weed looks like it may be Japanese stiltgrass http://extension.umd.edu/hgic/japanese-stiltgrass
not nimblewill http://extension.umd.edu/hgic/nimblewill See our website for more information.
If the weed is stiltgrass, you will have to renovate your turf in mid to late August. You will have to kill the turf and weeds using a nonselective herbicide such as glyphosate. Select a turf type tall fescue grass seed. See our publications and video for step by step renovation information. Begin by testing your soil now.
Results give pH, liming, and nutrient deficiencies. http://extension.umd.edu/hgic/soils/soil-testing
Next spring, you can apply a preemergent without nitrogen fertilizer. Look for the active ingredient: Prodiamine (Barricade) or other preemergents labeled for crabgrass control. See more on the stiltgrass link above.