It may well be that you do not have to disable the onboard graphics just install the new card and see that it is installed. If so shut down and move the cable from the onboard graphics to the pci card.
If the system objects then Reboot and press the key listed to go into the bios it is often delete or F1. just enter each page in turn until you see the onboard graphics entry then press usually page up/page down to change the entry it will probably say enable/disable. Press esc to shut each page and then click on save changes and exit.