Click on the Distribution list name form your main contact list, you will be able to tell the distribution lists apart from your contacts as there will be no details shown in the contact data. To edit/update the list click on it's name.
If you would like to add a new contact to the distribution list then select it from the list on the right side of the screen.
To remove a contact from the distribution list, click on the name you want to remove in the members list. This will remove it.
To delete the entire distribution list click on the "Delete" button
You will be asked to confirm the deletion of the contact list, click "OK" to delete the list.