How does RFC 5652 authenticated data confirm origin and data integrity?
How would receiver know that it was not from originator?


The recipient doesn't know the sender with this scheme. It can be anybody.
Why? There's no secret involved in the sending process that is unique to the sender and thus anybody can create such a message, as you already noted.


  Why can't an attacker do this?


He can. However this looks more like a building-block for other schemes and as such these may implement advanced sender-authentication mechanisms which would prevent such an attack.


  How would receiver know that message has been changed?


The recipient always knows that the message is unaltered with respect to the sender, because the MAC is valid. The fact that the sender may change is not covered with this scheme (and in fact, an attacker does change the sender to himself with the obvious attack).