Updated for 7.0
On the left-hand side of the screen in the navigation bar under Events, select Carnival Year. This will take you here:
To edit the carnival year dates, click the Edit button at the top of the screen on the right-hand side.
A calendar will pop up that looks like this:
Click the new desired date.
When done, click the Update button to save changes.
This only applies to editing the basic information for the existing Carnival Year.