In addition to loosening the screw that's inside the hole located under the shotgun barrel (you have to remove the barrel set by breaqking the gun open, depressing the detent on the side and pulling up on the barrels), you'll need to remove the screw on the left side of the front sling swivel and slide the sling swivel out so the forend can move over it.
Putting the barrel set back into the receiver is a little tricky and not intuitive. You have to grasp the barrel set and push back against the receiver firmly to lower the barrels back into the slots on the front of the receiver so they'll slide down and the detent can reseat.
Hope this helps.
I have two Brno combinations guns, a ZH 104 in 5.6x50R Magnum over 12 gauge with a set of 7x57 over 12ga and a ZH304 in 22Savage Hipower over 12gauge. Both are great guns; very accurate and super fun to hunt with in the woods.