Second You Must Have Pc With Android platform tools (They Exist Also For MacOS And Linux)
First thing to do: unlock the bootloader (if you didn't already do this, obviously). Smartphones with Android One are much easier to unlock compared to other Xiaomi phones with MIUI (where you have to ask for authorization and wait for weeks).
CAUTION: when you unlock the bootloader, the phone will reset, erasing all your saved data. Backup your data before unlocking.
1. Go to Settings > System > About phone > and tap many times on "Build number" until you unlock "Developer options".
2. Go back to the previous page, where you can find now "Developer options". Go there and enable the "OEM unlocking" option (and it's better to never disable this).
3. Now you can shutdown your phone, then turn it on while holding the "Volume down" button pressed. Release it when you see the Fastboot screen.
4. Now connect the phone with your PC via USB cable, open an administrative command prompt, move to the directory where the Android platform tools are placed and unlock the bootloader with the following command:
fastboot oem unlock
Now you unlocked your phone's bootloader and can continue to the next section
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open an administrative command prompt, move to the Android platform tools directory and then write the following command:
Fastboot boot boot_rooted.img
HOW TO APPLY OTA UPDATES
IMPORTANT: OTA updates will work only if all the partitions on the smartphone are untouched. Magisk Manager can restore the original boot.img following this section of the guide, but if you tampered directly with the system partition (e.g. manually editing build.prop) or other partitions, OTA will refuse to install.
You can try to fix OTA updates without the need to flash the whole original fastboot ROM, by flashing only the tampered partitions (usually only "system") with the corresponding img file found inside the fastboot ROM zip. Check the "Troubleshooting" section of this guide for details.
12. When you know that a new OTA update is available and you want to install it, open Magisk Manager, and tap on "Uninstall" > "Restore Images" but very important: absolutely don't reboot the phone now!
Important note: Magisk Manager will restore the boot.img that was found on the phone while installing Magisk. If the boot.img was already non-vanilla (for example you flashed the boot.img with TWRP before installing Magisk), Magisk Manager will backup that modified version of boot.img, and when restored the OTA will not work, as that's not a vanilla boot.img.
13. Close Magisk Manager and go to "Settings" > "System" > "System update" and install the OTA update. After the update is downloaded, a two-stages update will begin.
14. When both stages of the update process completed, it will ask to restart: DON'T DO IT!. Open Magisk Manager again and tap on "Install" > "Install" > "Install to Inactive Slot (After OTA)" and after that you can tap on "Reboot".
After the reboot you will have the updated Android version with Magisk already working.
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I Take Some Things From https://forum.xda-developers.com/mi-...t-ota-t3836952