Currently developers upgrade from ubuntu 16.04 to 18.04 by flashing the kernel by physically connecting the reference board and it requires reset and recovery lines. How do developers setup the upgrade to boot loader and kernel when the product is already sold in market. How to schedule and perform Ota for boot loader and kernel in such a situation .
you could clone the APP partition to update the ubuntu version.
however, since different JetPack release also have kernel upgrades (i.e. from 4.4 to 4.9), you must to update the kernel-dtb along with kernel partition.
please also refer to Topic 1049466 to do the trick if you would like to upgrade from JetPack-3.3 to JetPack-4.2.