- Find a firmware: (https://www.asus.com/Motherboards/Z87PLUS/HelpDesk_Download/)
http://dlcdnet.asus.com/pub/ASUS/mb/LGA1150/Z87-PLUS/Z87-PLUS-ASUS-2103.zip - Unpack it and localize Z87-PLUS-ASUS-2103.CAP (or some other older version Z87-PLUS-ASUS-2004.CAP or Z87-PLUS-ASUS-1802.CAP)
- Prepare a FAT flash device
- Copy the firmware *.CAP file to flash root directory and rename to: Z87PL.CAP
- Plug flash into the 'flashback' USB port at the back of PC (it's next to ethernet with a green border around)
- Press flashback button on a motherboard, it's below a power plug, labeled as "BIOS_FLBK"
BTW. It sucks that Asus just doesn't say what should be the name of a firmware file. They could just use FIRMWARE.CAP and it would be fine. There are other ways of protecting people from themselves than encoding the model in a file name.