Snappy is a command line tool for manipulating APFS snapshots on applicable *OS versions.
If you are on tvOS 11 and want to perform an OTA update on a device that is currently jailbroken you will need to revert to the original snapshot before continuing.
Reverting to original pre-jailbroken filesystem
1. Install Snappy (com.bingner.snappy)
2. SSH in to your Device (ssh root@DEVICE_IP)
2. snappy -o
You may need to unblock updates a few other ways if you have installed any kind of profiles to block them (ie watch os beta profile)