First, enable edit-mode by clicking on the pencil on the top right. Then move the mouse over a gantt bar:
- within the bar, you can drag'n drop the whole bar left/right into the past/future keeping the duration in working days
- on the left side of the bar, you use drag'n drop to extend or reduce the duration by adjusting the planned start date
- on the right side of the bar, you use drag'n drop to adjust the planned end date accordingly
All depending issues as well as subtasks are re-scheduled automatically, if you have enabled "re-scheduling" within the add-on's global configuration (which is the default).
You need to have schedule-issue permissions in order to modify planning dates by drag'n drop of gantt bars or shifting of planned start/end dates.