In-line editing of elements of the Gantt chart is possible. Start and end dates can be edited by clicking into them, if shown. Click the gear icon to choose to show / hide date columns:
Note that changing the start date will not change duration (the end date will move as well), whereas changing the end date will change duration (start date will not be changed).
If tasks are being shown in your Gantt you can enable re-ordering of tasks. Doing so allows you to drag and drop bars in your Gantt as you wish - your mouse cursor will change to a four-point arrow to indicate drag and drop is allowed. The Gantt does not support re-ordering of phases.
Descriptions of phases cannot be edited in-line due to the multi-line nature of phase descriptions. Note that if you are displaying tasks in your Gantt chart, the descriptions can be edited in-line as these are only stored as single lines of text.
You can also add new items to the Gantt from withing the Gantt view. Phases and tasks can be added by clicking the + icons: