Continuing the discussion from LineageOS on Galaxy Tab S6 Lite:
In order to upgrade I needed to update the recovery image.
- First, follow instructions at
https://wiki.lineageos.org/devices/gta4xlwifi/upgrade
to turn on USB debugging and verify the device connection - follow the section at
https://wiki.lineageos.org/devices/gta4xlwifi/install#installing-lineageos-from-recovery
- Return to the upgrade page, and
adb sideload $update