In the Manage Adverts page click the Options icon under Advert Groups and select the tab called Delete/Rename a Group. This will bring up a list of your active groups.
To rename a group
1) Click on the group name you wish to edit.
2) Type in the new name.
3) Click ok.
To delete a group
1) Click the red cross icon to delete the group (this does not delete the adverts within it).
2) Click ok