How is decryption done in AES CTR mode?
A slight correction about terminology:


The key is constant when you use CTR.
The IV/counter affect the cipher input and so the keystream varies.


The reason this can be decrypted is that the decrypter knows both the key and the IV/counter. They can calculate exactly the same function as the encrypter did, resulting in the same keystream block, which a XOR then cancels out.

The images on the wiki page for block cipher modes show how both sides compute the same thing.