You'll need to post a lot more details?

Hi, welcome to GoY, it's not a good idea to lay new turf on top of old, as you must have a reason for wanting to lay new, maybe the old turf is full of weeds, which if you laid new turf on top, would still be there, and come through very quickly, far better to remove the old, and prepare the ground from scratch, Derek.

Agree, if you want a consistent lawn like a green carpet effect, you have to pull up and discard all the old stuff, recondition the soil, then lay out the new, either seed or turf. Avoid using weed killer or toxic chemicals which can linger in the ground and damage the new grass.

if you have the space to allow the old turf to rot down do so. the soil is usually good for topping up borders. lay the old turf grass side down in layers.

once you have sorted the area then lay your turf.
the rhs advice for the uk is usually very helpful.

https://www.rhs.org.uk/advice/profile?PID=410

welcome to GoY too :o)

If you are looking to re-turf the whole lawn and it's a large area, I would consider hiring a machine to cut the old turf in large sections. Then if the old lawn was level, I would just rake the soil to produce a fine tilth of an inch or so rather than dig it over and try to re-level. If there are some deep rooted perennial weeds, I would look to kill them with Glyphosate or hand weed. If on the other hand you feel that there is still some life in the old lawn, I would consider using a selective weed killer and then re-seed. Once April arrives we should have some warmer but humid weather and new grass sowings should really take off.