Install the Android Debug Bridge (ADB)
https://wiki.lineageos.org/adb_fastboot_guide.html
https://github.com/M0Rf30/android-udev-rules#installation
# check if device is found
adb devices
# reboot into sideload modus
adb reboot sideload
Or manually boot into TWRP recovery, holding Volume Up + Power when the phone is off. Navigate to Advanced-> ADB Sideload.
Update MIUI Firmware
If your current MIUI Firmware does not support Android 10, you have to update it first.
Download the newest MIUI Firmware for your device from https://xiaomifirmwareupdater.com/firmware/dipper/.
Flash the new Firmware via TWRP or via ADB sideload.
adb sideload Downloads/fw_dipper_miui_MI8Global_V12.0.2.0.QEAMIXM_4e38c51916_10.0.zip
Download and flash new LineageOS image
I’m using the LineageOS fork LineageOS for microG. Download it from here: https://download.lineage.microg.org/dipper/ (MI 8 = dipper)
The upgrade steps are the same as for the official rom: https://wiki.lineageos.org/devices/dipper/upgrade. In my case only flashing the new image.
adb sideload Downloads/lineage-17.1-20210327-microG-dipper.zip