Please use below steps to update bios.

a. Boot to BIOS setup.
b. Set "Me FW Image Re-Flash" to ENABLE under Advanced->PCH-FW Configuration->Firmware Update Configuration page.
c. Save and exit the BIOS
d. Boot to the bootable USB/CD/DVD media that contains the BIOS flash package.
e. To start flashing BIOS, from this step, refer to "Readme for AMI BIOS.txt" Step1~Step 6.
f. After the update finished, restart the system.