When your system starts up - and (hopefully) is displaying the bit where you get white text on a black screen - then normally you would (repeatedly) hit the Del key - this then enters the BIOS and gives you several text screens with many settings for your asyatem in it. The instructions for moving among the screens, and how to change settings, and how to save the changes, etc - will be on the screen along with the settings.
Sometimes you don't see the initial text screens, but instad a manufactureres log - if so try the Esc or Tab key first.
Sometimes the key to enter the BIOS is not the Del key - wich key it is is also shown on the screen.
If you have a manual for the motherboard in your system, it will almost certainly have instructions on how to enter and change the BIOS.