When you are building a loop and you are in the edit mode, you can choose to delete a hub. When you do this, all nodes connected to the hub will also be erased. Open the hub panel and click the trash can and Erase at the bottom of the hub panel. There is one of your hubs that will not be erased and that is your Start hub. If you want to be able to erase it, you first have to turn one of your other hubs into the Start hub.
