To edit an existing group of elements:
Select your group in the Object Browser and in the Mesh menu click the Edit Group item or "Edit Group" button in the toolbar.
The following dialog box will appear:
In this dialog box you can modify the name of your group and add or remove the elements forming it. For more information see Creating Groups page.
To convert an existing group on geometry into standalone group of elements and modify:
Select your group on geometry in the Object Browser and in the Mesh menu click the Edit Group as Standalone item.
The group on geometry will be converted into standalone group and can be modified as group of elements.