Welcome to the forum!
You do not need to rename the cap file, just placed it (and nothing else) on a formatted FAT32 flash drive.
Place it into a USB port, start the PC tab the delete key to enter the BIOS.
Navigate to Tool, then select ASUS EZ Flash 2 Utility and follow the simple guide.
Easy as 123 and no CPU is needed.
No you don't need the CPU installed
Yes it'll flash it, that's what the ez flash he mentioned will do, it needs to be a freshly formatted flash drive with nothing but the bios and the ez flash should auto detect it, you probly dont even need to rename it, the ez update thing will show you what it is when u select it or when it auto detects it
If you wanna see visuals JJ from Asus has YouTube videos on bios updating, his channel is PCDIY or Asus PCDIY
Also if I were to take a stab in the dark at the warning message ur asking about, around the last bios they released a bios updater that would run thru the OS, my guess is the warning is directed at ppl upgrading from haswell to haswell refresh letting them know they need to update or that maybe by having the app it would tell you in first boot that you need an update, not an expert or anything here just a shot in the dark guess lol
I believe but I'm not really sure so I'd wait for another to confirm but it should be just the main 20+4 or 24pin power