In order to remove upper receiver, you must first push out the body pins. (The body pins do not come all the way out.)

It is recommended to use a punch that is slightly smaller than the body pin. Once the body pin is sticking out half way you should be able to pull it the rest of way by hand.

Do not remove the pin completely. The pins are meant to stay attached to the body body so they wont get lost or fall out on their own.