Downgrade to previous firmware version

I have At&t Samsung Galaxy Tab E 8.0 (T377A). Today its firmware is updated from 6.01 to 7.1.1.  As I am using this device to develop and test a corporate Application which is Android 6 only.

So my question is, how can I downgrade to previous version? I don't see any stock firmware from At&t to download.

Thanks,

Raby