You’ll need to apply a moss killer.  Then, after the moss is dead, you’ll rake it all out and broadcast grass seed to fill in the area.  You’ll also want to do a pH test and apply enough lime to correct the acidity level in the soil.  Acidic soil is the reason the moss took over.  Grass dies off as the acidity of the soil increases over time and the moss, which prefers acidic soil thrives and chokes out the struggling grass.  HERE’S A LINK that will walk you through a simple pH soil test:  https://blog.timesunion.com/gardening/the-acid-test/567/