You will need the Full Write Groups permission to edit a group.
Click on "Groups" from the left hand navigation and select the group you want to edit from the listing by either clicking on the name or on the "View" button at the end of the row. Then click on "Edit group" from the "Actions" menu in the top right area.
A pop up will appear where you can change the details of the group. See this article for information on the available fields.