As the motherboard is a custom Sony board, you can only use BIOS updates supplied by Sony. Unfortunately, Sony BIOS is limited in content and locked down by Sony. Try any other BIOS and you may end up with an expensive doorstop!
Why do you need a BIOS Update?
There is a BIOS update R0190Y9 available on the download site for your model which adds a CPU C3/C6 switch to the BIOS Setup Advanced Menu and also fixes some USB keyboard compatibility issues - look under Windows 7 Latest Downloads here: -