Question about upgrading to Ubuntu 16.04

There is alert message from os(Ubuntu 14.04 that Nvidia support) like ‘It is available to upgrade Ubuntu 16.04, you want?’

Is it recommend to upgrade to 16.04LTS from a way from os?

There is no automated note on upgrade (you have to ignore Ubuntu-specific upgrade notes since the purely Ubuntu upgrade does not have required hardware drivers). The L4T R21.5 release is the most recent L4T release, found here:
https://developer.nvidia.com/linux-tegra-r215

Unfortunately this is Ubuntu 14.04 and not 16.04 (the “L4T” release is just NVIDIA drivers on top of what is otherwise just plain Ubuntu…R21.5 is ubuntu 14.04).

I do not know if there are any plans for a future TK1 L4T upgrade to Ubuntu 16.04.