Thanks, EskeRahn and John Veness,
I entered Settings -> Apps -> See all ** apps -> (...) show system, and cleared cache/storage for two "Wireless Update", and reboot.
Yes, I could enter "settings" of system update just once after cleared chaches/storages, however, it went to white-blank screen again when entering twice...
And (unfortunately) it still says "your software is up to date" :- (
PS:
I also found "SD card update" option in settings page, it may be one hope before manual flash.
Dows anyone know how to get update package of 20200825 that can be used for "S