To remove the hip belt, you will need to loosen the velcro and pull the hip belt out until its completely removed.  You may need to place your hand inside the opening to prevent the velcro from re-attaching.
To re-insert the belt you'll need to hold the opening open, and avoid letting the velcro close on it as you slide it in. It might help to open it and slide it in until you get to the point that you can reach it from the other side. Then switch over there and pull while keeping the insert side open. Once it reaches its spot, let the velcro go and it's in!