Thank you for replying.
Yes, I tried that.
I downloaded on a usb drive, from asrock, the BIOS 13.02 and 14.02 for B660M-STX, also BIOS 9.02 for B660M-ITX/ac (to test if the BIOS flashing is working). Then according to instructions Tools / Instant Flash:
It recognized only the version BIOS 9.02...