HOW TO overseeding an existing lawn.my old grass is dry and ugly?

Question:PLEASE HELP URGENT I NEED TO KNOW

Answers:
You have a couple of steps to go through. It sounds like the rototiller may be a bit more that you want to do so I will assume we are keeping work to the minimum but are working with a yard say 60% dead.

Cut the grass short.
Rent a Thatcher and run it over the lawn a couple of times. Get as much thatch up as you can. Your lawn mower will pick up a lot of it, a Rake for the rest.
Rent an Airator (Hole puncher). Run it over with the plugs set deep.

Water heavy a couple of days in a row to get a good soaking of the soil and then seed and apply a fertalizer called started fertalizer. It has a chemical to promote sprouting. You will have to keep the young seed moist so you will have to water a couple times a day for about 2 weeks.

The dethatching and airating give the seed places to establish themselves and the fertalizer will kick it in gear.

It should be a rich lush lawn in 3 weeks or so.