For the 2A99, the most recent stable update is often 6.14 Rev. A (released circa March 2011). Preparation:
Boot from the USB, enter the DOS environment, and execute the flash command. Warning: Do not interrupt power during this process, as it will brick the motherboard. Common Issues & Troubleshooting pegatron corporation 2a99 motherboard drivers upd
If the "Device Manager" shows yellow exclamation marks, use the "Update Driver" option and point Windows to a folder containing unzipped legacy Windows 7 drivers. For the 2A99, the most recent stable update is often 6