How is HMAC hashed message decrypted on the receiver side
Actually the HMAC value is not decrypted at all. The recipient takes all the needed input and she computes the HMAC on her own side and check if the result she got it is equal to the value on the message she got. 

You can roughly see the HMAC algorithm as an symmetric key signature. You cannot decrypt an HMAC, you only check that the value is correct.