The brass insert should just unscrew from the new spout but, typically the old brass insert is just threaded on to a male threaded adapter that is sweated on the copper line off the valve and it should unscrew.
Try gently with a couple of channel lock pliers to remove it from the spout connection.
Post a pic of the piping
Try gently with a couple of channel lock pliers to remove it from the spout connection.
Post a pic of the piping