hello denma0513,
please see-also developer guide, Updating Jetson Linux with Image-Based Over-the-Air Update.
since you’re based-on JP-5.1.2/r35.4.1, you should first upgrade to JP-5.1.3/r35.5.0, and then from r35.5.0 to do OTA update moving forward to the JP-6 public release versions.
it’s suggest to update l4t_generate_ota_package.sh and ota_board_specs.conf for customize board to create OTA payloads.
see-also similar forum topics for reference, Topic 332980, Topic 349289.