how would I update the BIOS if I can't get it to POST?
Well, that's the thing. You can't. At least not without a Sandy Bridge CPU installed. There's no way to do it with an Ivy CPU installed.
You have 3 options, though. You can buy a cheap Sandy Celeron, you can borrow a Sandy CPU from someone, if someone you know has one (or even a local computer shop), or you can just return the Z68 board and get a Z77 board.
Personally, I would probably just return the Z68 board and get a Z77 board.