DOS is a text-only operating system, not a part of the computer. Windows is an operating system also but is graphics-based and much more complicated. Your computer's BIOS is software that is built into the computer with a hardware chip and exists outside operating systems like Windows and DOS. How you get into you BIOS is always the same, but doing it from DOS will be a bit simpler and quicker because DOS is so small.
Exit DOS by hitting the reboot button on your computer.
Watch the screen for a listing of what key to press to enter the BIOS. Usually this will be at the top or bottom of the screen and the key is often DEL, or a function key such as F2 or F8.
Press the key quickly. Sometimes you may need to press the key several times to get it at just the right time. Intermittently pressing the key usually does the trick. The BIOS will load and you will be able to make changes to the BIOS.