On an issue page, click the title or description to edit it. Choose a new status, priority or assignee from its value, or click the due date to pick another. Each change saves as soon as you make it, and returns to the previous value if it cannot be saved.
In the description, Enter starts a new line. Press Ctrl+Enter (Cmd+Enter on a Mac) or click elsewhere to save, or Escape to cancel. Labels, milestone, cycle and custom fields are still changed on the separate edit page, linked below the properties.