Download the BIOS from (the most recent is the "Z87-A BIOS 2103") http://www.asus.com/uk/Motherboards/Z87A/HelpDesk_Download/
Copy the file to the root of a USB Drive
Shut down your PC, then power up while pressing the delete until you enter the BIOS.
You may need to enter advanced mode, to do this click the button in the top right labeled "Exit/Advanced Mode" (or something similar).
Use the left and right keys (or mouse) to navigate to the "Tool" section of the BIOS
Press Enter, then select ASUS EZ Flash 2 utility
Select your USB drive, then the BIOS file and confirm that you would like to update the BIOS
Here is a video for your reference:
Edit: You may also find page 52 useful:
http://dlcdnet.asus.com/pub/ASUS/mb/LGA1150/Z87-A/E7828_Z87-A.pdf