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why do you want to downgrade?
did you look at Sammobile
here is the firmwares they have for you:
https://www.sammobile.com/samsung/galaxy-tab-a/firmware/SM-T580/
I don't think that Android 6 Marshmallow was ever made for this tablet, it's too recent. it came with 7 Nougat when it was new AFAIK. If you really wanted 6, it would need to be custom built.
Have you got some old app or game that won't run on anything later than 6?
I seem to recall a previous thread where you asked about Flash problems, so I've deduced that this is an important issue for you.But i have a problem with the flash files on 8