Lines 1,2,3,4,...,n-1, n to lines n,n-1, ...,4,3,2,1

I just realized that I could solve this problem by reversing line numbers from 1, 2, 3,...,n to n, n-1, ..., 3,2,1 and then use the same logic as earlier. So I want to know how can I reverse the order of lines?