Ubuntu 16.04 on Jetson TX2 via SDK Manager

how can i install Ubuntu 16.04 instead of 18.04 via SDK Manager?

I just bought TX2 with Ubuntu 16.04 on it and unintentionally have installed 18.04 via SDK Manager.
How can I get back to 16.04? I need it because of ROS Kinetic.

The JetPack3.x series is Ubuntu 16.04, while the 4.x series is Ubuntu 18.04. See JetPack3.3 here:
https://developer.nvidia.com/embedded/jetpack-archive
(you might need to log in there and go to the URL again)

Btw, don’t mix packages from different JetPack releases. If you use JetPack3.3 for your o/s, then use its version of CUDA and don’t mix with something like JetPack4.2.