How does changing transaction data in a block change the hash of previous block?

How does changing transaction data in a block change the hash of previous block?

I am reading this article, within which it mentions that

"Imagine this for a second, a hacker attacks block 3 and tries to change the data. Because of the properties of hash functions, a slight change in data will change the hash drastically. This means that any slight changes made in block 3, will change the hash which is stored in block 2, now that in turn will change the data and the hash of block 2 which will result in changes in block 1 and so on and so forth."

And I do not understand if a hacker attacks block 3 and try to edit the transaction data within it, how would this affect the hash stored in block 2? And hash of what is stored in block 2 ?

Can someone kindly clarify for me please ? I am just getting confused.

http://ift.tt/2Axrbty