In a Blockchain how difficult is it to modify the third to last block

Question

Let's say my Blockchain has N no. of blocks. My question is - how diffcult is to edit (N-2)th block on the PoW Blockchain?

Answer 1

Technically, it's not difficult at all, all you need to do is recalculate all of the entities after the one that was modified. This is straightforward and inexpensive. 

Although, that will give you five new hashes, so everybody immediatelly notices that someone has manipulated the Blockchain. that is one of the fundamental features of PoW Blockchain.