To edit a the properties of a Media Group go to Explore > Media Groups. As you mouse over the Media Group in question select the Edit Media Group Properties option.
The following Media Group properties can be edited at any time:
Name (max 50 characters)
Publicly Viewable
Password (optional)
The folder they are saved into (optional)
Whether this is your Active Media Group
Description (optional)
If there are images with annotations that have been saved to a specific Media Group and you change the name of that Media Group, the annotations will not be lost, they will still be associated with that specific Media Group