The Xiaomi Mi MIX FOLD official firmware on this page will help you update your phone MIUI version to the latest. To move from Beta to Stable ROM (vice versa) or un-brick your phone, you need Fastboot ROM.
If you previously rooted or modified the system partition of your phone, you should consider flashing the Fastboot ROM. It will overwrite all existing modified system files to replace with factory software. To ensure that all important data on the phone (like contacts, messages, app data, etc.) is safe, back them all up because you could loose data after flashing.
Again, charge the phone to at least 30% so that the flashing process will not be interrupted by low battery. Interrupting the firmware installation means bricking your phone.
Download either the Fastboot or recovery firmware from the link(s) below and follow the recommended flashing instructions immediately after the download options. Do not hesitate to report any broken links detected.
Xiaomi Mi MIX FOLD official Fastboot ROM
Xiaomi Mi MIX FOLD Recovery ROM (local update)
Important! Always backup existing firmware before flashing any firmware.
How to flash Xiaomi firmware
After downloading the firmware from above link, get a functional USB cable, then follow the instructions via this link to learn how to flash Xiaomi Fastboot and Recovery firmware.
If you have any questions or other contribution please post in comments section for assistance.