When you log in to your Channable account you will see one or more projects there. Before you can delete a project first you must deactivate it. You can do this by going to Settings > Project settings and clicking on 'Deactivate project'. This will also deactivate any remaining active channels in the project.
To delete the project, now go to Settings > Project settings in the project you want to delete and click on the cogwheel in the top right. Here you can click on the option 'Delete project'.