Close the door as fully as possible and pull down on the emergency release handle. The emergency release handle is the cord that's dangling down from the trolley. Pull on the handle until the trolley release arm moves into a vertical position.
Lift the door manually. Close the door manually to test.
Re-engage the trolley when you're ready. Pull on the emergency handle toward the opener at 45 degrees so the trolley release arm is horizontal.
Open the door by pressing the open button and the trolley will reconnect during its first opening. Test the door by pressing the close and then open buttons.