Update LineageOS on Xiaomi Mi 9T Pro/Redmi K20 Pro (raphael)
Using the LineageOS Updater app
Open Settings, navigate to System, then Updater.
Click the Refresh Icon in the top right corner.
Choose which update you’d like and press Download.
When the download completes, click Install. Your device will reboot to recovery and install the update, then reboot to the updated installation.
Sideloading from Recovery
Download the LineageOS zip file. Simply download the latest zip file, named lineage-21.0-xxxxxxxx-UNOFFICIAL-raphael.zip.
Make sure your computer has adb and fastboot installed. Setup instructions can be found here.
Enable USB debugging on your device.
Now reboot into recovery. With the device powered off, hold Volume Up + Power. Keep holding both buttons until the “MI” logo appears on the screen, then release.
On the Lineage Recovery home screen, tap Apply update, then Apply from ADB for starting the sideload service.
Open a ADB & Fastboot tools window on your PC and flash the LineageOS install package you downloaded above: