If a lawn has deteriorated badly due to for example pest infestation, drought or simply neglect it may be necessary to renovate.
If the problems are severe you may want to consider new turf. Whilst this will give a quick result it is quite costly and new turf does not always adapt well to its new environment whereas seed acclimatises as it grows.
A lawn renovation on a pretty poor lawn is likely to be as follows:
Treat the lawn with fertiliser, moss control and weed control.
Mow the lawn short.
Scarify the lawn hard. Clear debris.
Aerate the lawn heavily with a hollow tine aerator.
Spread seed on to the lawn.
Top dress the lawn if needed.
To recover from this work the lawn will require moisture and warmth so the best time to do such work is during Autumn. It will be necessary to water the lawn frequently in the absence of rain.