How to upgrade to L4T 21.3?

How can I upgrade my current L4T 21.2 to L4T 21.3 without doing a complete reinstall.

Unfortunately there are no instructions for doing that. You would need to update at least the U-Boot, kernel, DTB and user-space binaries. The last three are easy but I’m not sure about the U-Boot. And I’m not sure either, if I’m missing a component here.

I updated from L4T 21.2 to 21.3 with following commands.
They don’t update U-Boot, but I can boot Ubuntu.

sudo tar -vxjf Tegra124_Linux_R21.3.0_armhf.tbz2
cd Linux_for_Tegra
sudo ./ -r /
cp /boot/extlinux/jetson-tk1_extlinux.conf.[your boot device] /boot/extlinux/extlinux.conf

Check driver files

sha1sum -c /etc/nv_tegra_release

Reset Jetson TK1
Login to Jetson TK and check kernel version

uname -a

Then, I installed “The Grinch” Custom Kernel 21.3.

It seems when jetson tk1 boots, U-Boot reads /boot/extlinux/extlinux.conf from the boot storage and load a kernel with parameters specified in that file.
So, I think you don’t need to update U-Boot unless your current U-Boot cannot read extlinux.conf from your storage or you want new futures in new U-Boot.