Cipher Block Chaining Ciphertext Alteration
One way is simply to look at the usual illustration of CBC decryption and follow the arrows.
Here is the figure from Wikipedia:



Imagine you alter the first block of ciphertext. Follow the arrows to see what will be altered in the plaintext: only two blocks.

We can also do this with math formulas:
CBC mode encryption can be described as follow,
where $ P_n $  and $ C_n $ are the $n$th plaintext and ciphertext blocks, respectively:

$$ C_n = Enc(P_n \oplus C_{n-1}) $$

(With the IV playing the role of $ C_0 $)

Thus 

$$ P_n = Dec(C_n) \oplus C_{n-1} $$

As a result, if you alter $ C_n $ it is clear that
$ P_n $ and $ P_{n+1} $ will be affected but not $ P_{n+2} $ and the next blocks.

If you want to see this in practice, you may want to do the cryptopals challenges up to challenge 17.