Installing older versions of firmware doesn't always work. And the bigger the difference in version, then less likely it is. When going back, you should do a factory reset and then use the Kobo desktop to do the setup and intercept the firmware update. The desktop application will do this, but, you can replace the update files with the files for your desired firmware version.
In all probability, you will be able to do a manual factory reset to get the device going. The instructions are on
https://help.kobo.com/hc/en-us/artic...our-Kobo-Touch. But, in case that doesn't work, I have sent you a PM with the link to the image.