The answer largely depends on what make of computer you have, as different manufacturers use different options.
That said try pressing the F2, F10, F12, or the delete key continuously as soon as you power up you PC until you see a message indicating that the bios is starting or the bios screen appears.
WARNING: be careful when changing settings. Always make a note of what changes were made in case you need to undo anything.
A Mac's "BIOS" is not really a BIOS. It is OpenFirmware, which many Mac users find to be better than BIOS. To access OpenFirmware, you must use two hands on the keyboard and the key combination CMD-OPT-O-F.
The delete key is also used to access bios on some systems.
By pressing certain keys (depending on the specific computer and BIOS program) during the boot process, ex: F12, F2, F8,
try to press alt f10
In the BIOS setup program.
The BIOS Manufacter
FN + F1 > together
Assuming you mean "flash the CMOS", when the CMOS settings are so messed up, you cannot access the BIOS.
The Del key
F1 or f2
When you FIRST power on your computer, you should see a flash logo or a flash screen that lists your brand of BIOS. You may be able to push DEL or F12 on initial boot to access the BIOS settings.
One password is for administrative access to BIOS settings. The other is optional and is used for unlocking input devices (particularly, the keyboard) on startup.