Unscrew the magazine end cap. The barrel and forearm must come off together as a unit - pull straight out.
To remove the forearm, hold the barrel extension with left hand, and with the right hand push the forearm slightly towards the muzzle while pulling away from the barrel. Once it comes free, pull it towards you and off the magazine ring.
On most guns, you can pull the forearm off with the barrel still in the receiver. Not this gun. The forearm is captured by the barrel magazine ring.