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)
The folder they are saved into (optional)
Whether this is your Active Media Group
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