A group will only show in the Category Manager when it contains categories, so in order to edit (or even delete) a group it must contain at least one category.
When a group contains a category it will show in the Category Manager similar to below.
To rename the group just click the group name to see the following popover.
To delete the group entirely hit the trash can symbol shown towards the right of the row. This will remove the group and unbundle any categories contained in the group. The categories will revert to showing individually in the cash flow.
If you're looking to edit the actual categories, not the group, check out how to here.