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For Switching from Beta to Stable ROM and Vice-versa or Flashing to an Older/Discontinuous ROM Version, You have to Unlock your Device's Bootloader first and then Fastboot Flash with the respective Fastboot ROM Version Available for your Device Via Mi Flash Tool and PC.
Refer this thread for Unlocking Bootloader : https://in.c.mi.com/thread-132126-1-1.html
For Fastbooting your Device, Download the Respective Fastboot ROM and Follow the Guide :https://in.c.mi.com/thread-324489-1-0.html
Take Necessary Backup of your Important files and Data before Flashing as it will Full wipe your Device.
If you want, You can Re-Lock the Bootloader via Mi Flash Tool while Fastbooting. Select Clean All and Lock option under Mi Flash Tool to Lock your Bootloader.
Please Note that You must Do Fastboot Flash While Flashing to an Older ROM Version or Switching between Beta and Stable ROMs. Else it might lead to serious App Force Close and Other Major Issues and may even brick your Device.