Project > List
With a project in context, click on the Edit icon (pencil) to the left of the corresponding role you wish to modify.
In Edit mode, you will have the ability to Edit the Start date and End date of the role to your liking.
Note: Role dates must be within the start and end dates of the project in question
Make sure to click SAVE once you're finished editing.
Project > Gantt
You also can edit Role dates from the GANTT view as well. Similar to above, this is accessible under the Edit button to the left of the Role. Don't forget to click Save to apply your changes!