Caution: Data will be encrypted And This process can brick your device. Don't blame me
~ You need to have unlocked bootloader
~ Download any AOSP A/B build
~ Download vbmeta attached below
~ Goto Bootloader, enter
fastboot --disable-verity --disable-verification flash vbmeta vbmeta.img
fastboot flash recovery recovery.img
~ Wipe Dalvik, System, Data & Cache.
~ Extract the .img from downloaded AOSP file *.xz & *adb push *.img /external_sd or whatever directory
~ Locate the extracted file & install it through twrp. Reboot to System.