You can use old grass after
lawn renovation in two ways. If the grass is still alive, just roll it in places where the grass is rare. It will settle and help to even out the emptiness. If there is a lot of old grass or weeds, cut it into pieces and add to compost. In a few months, you will make a great organic fertilizer. The main thing is not to leave a pile on the ground - there will be pests. Better to use or recycle.