How to upgrade ThinOS correctly

As I got more and more request on how to upgrade ThinOS device the correct way, I thought it would be a good idea to write a small summary.
I will not explain how to upgrade the clients technically using a fileserver with /wnos folder, etc. This is something I assume you all know.

ThinOS is designed to upgrade or downgrade from any version to any version. However there are a few exceptions:
1) Units with new Dell Mac Address ranges require 8.1_027 or higher code. In newer 8.4_108 code there is actually blocking to prevent downgrades.

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