patching old grass up

hi all. my garden lawn is reallly un even and i was wondering can i just put top soil on the un even parts and re grass or do i have to take the turf off and then add the soil to fetch the level up then re seed ?, cheers for advice in advance

 

It depends on how uneven your lawn is Noksucow. Would you have a picture or two please?

 

pics please............