You need a pendrive (remember to backup everything inside it).
Use this guide ("Use Rufus"): http://www.howtogeek.com/136987/how-to-create-a-bootable-dos-usb-drive/
After format your USB with DOS, mount it and copy the files to update the BIOS to the pendrive. Unplug the pendrive, restart your computer, access the BIOS, plugin the pendrive and boot throught the pendrive. It should load the DOS.
Now, go back here to page 10:
http://www.tomshardware.com/answers/id-2093279/update-gigabyte-z87x-ud3h-bios.html
The display will be black and with a prompt, something like: A:/ or D:/
Use 'dir' to list all files inside the pendrive. If it contains the new BIOS version (maybe 'Z87XUD3H.F9'), then you type it name and press "Enter". Read it from the PDF: If the flash utility cannot be executed after system runs boot files, please try to
run the command: Flash utility BIOS file name under the DOS prompt A:/
For example: A:/FLASH864 7VRXP and press Enter.
Just read the guide!!
http://www.tomshardware.com/answers/id-2093279/update-gigabyte-z87x-ud3h-bios.html
Page 10!