Go to Dell's website and download a recent bios version for your model. These files come with own application that allows you to clear memory storing old bios and installs a new one, then restart your machine. BEWARE: installing wrong bios version may result in computer not starting properly!!
You will need to open the case on your computer. Consult your owners manual or look it up on the manufacturers website to find the reset bios jumper setting. Pull the shunt from the normal run position and place it in the reset position.
your dell computer should have came with a reboot disk and all you have to do is put the disk in and follow instructions if you didnt get one you can order them online