Shut down the computer.
After shutdown, press the power button.
As the BIOS screen flashes, you may see something along the lines of:
Press F2 for Setup
F12 for Boot options
F2 should take you into the BIOS. Note: not all computers are the same in this regard. Some computers require that you press Delete or Esc to enter the BIOS. If you miss it and Windows starts to load, just be patient while Windows boots. After Windows boots, shutdown and try again. Do not, I repeat, DO NOT do a hard shutdown while Windows is booting. This could render your system unoperable. Like I said, if you miss pressing F2 and Windows starts to load, just be patient and try again.