You can delete a folder from the folder settings.

In the mobile app, swipe left on the name of the folder then tap the "..." button. To access folder settings in the web or desktop app, hover over the name of the list and click on "...".

To delete a folder, go to your folder settings, and click "Delete Folder" in the "Settings" section. 

Please Note: Deleting a folder will delete all lists and tasks inside of the folder. This action cannot be undone. You can only access folder settings and the delete option if you own the folder.