Read those manuals:
Thanks Jock and paolosabatino!
You have to erase eMMC with multitool to be able to boot Armbian images from SD, then when you have preferable image, you can burn it to eMMC again with multitool. So before all you should make a backup of eMMC again with multitool. Even better try to find original android image for your box before erase eMMC. Extract DTB is also recommended.
Good luck!
PS, never opened this T9, as usually I open them only if I need to find test point with the oscilloscope, which mean: "things gone wrong"