ThinOS 9 Release Notes have been updated with downgrade details
If you are downgrading ThinOS 9.1.3112 to ThinOS 8.6, ThinOS 9.0, or ThinOS 9.1 previous versions, you must use the Dell Wyse USB Imaging Tool to install the ThinOS 8.x or 9.x Merlin image that is posted to www.dell.com/support. Before you downgrade, ensure that you disable the secure boot option, and clear Trusted Platform Module (TPM) or Platform Trust Technology (PTT) in BIOS. Else, ThinOS 9.0 resets to factory settings after each reboot.