How do I update the BIOS on the MSI B550-A Pro without installing a CPU or RAM?

You can update the BIOS using the Flash BIOS Button on the rear I/O panel without installing a CPU or memory. Download the latest BIOS file from the MSI website, rename it to MSI.ROM, save it to the root of a USB storage device, connect power to CPU_PWR1 and ATX_PWR1, plug the USB into the Flash BIOS Port, and press the Flash BIOS Button — the LED will flash during the process and turn off when complete. (Page 50)