Ubuntu 14.04 for armhf

How do I revert back to ubuntu 14.04 from ubuntu 16.04?

Also if I need to install ubuntu 14.04 where can I find it for armhf?

L4T R21.6 is the most recent available for a TK1. This is Ubuntu 14.04 with NVIDIA hardware-accelerated files on top of it. You can install this by flashing (anything going to 16.04 probably caused failure to access some hardware because those libraries won’t work on 16.04).

Flash can be from any x86_64 Linux host if using the driver package plus sample rootfs. Flash requires an Ubuntu 14.04 or 16.04 host if flashing from JetPack. The most recent JetPack which supports the TK1 (and is a front end to flashing) is R28.1 (the R28.2 is a pre-release found in JetPack3.2…which is only an early test release).

See:
https://developer.nvidia.com/linux-tegra-r216

Note: Flash only requires the micro-B USB cable. JetPack will flash over this, and then after the Jetson reboots use wired ethernet for exta packages…thus you also need wired ethernet to the Jetson if using JetPack to install additional packages.