Very simple to install 9.1 on mate 20 Pro (Lya - L29)
!!! You need a memory card !!!
!!! Be careful this action will relock your bootloader so don't forget your code !!!
!!! Be careful this action will erase your data !!!
1st- download the latest firmware:
In my case C432 (for europe) here: https://androidhost.ru/Vjh?pt=TkVvcl...VzNhUWh2ND0%3D
2nd- copy the "dload" folder on the firmware and past to your sd card
3rd- on your phone you have to write this: *#*#2846579#*#*
choose Software Update
Then Memory card Upgrade
your phone reboot with 9.1