As in most cases, as soon as you have picture on screen (in your case the Dell logo), try pressing F2 several times to make sure the PC gets the message. In new systems especially with SSD disks you can never be fast enough to enter BIOS, so to make sure you can, hit the F2 (or whatever button it is) as soon as you see a picture on screen. I hit it several times earlier, since some computers seek for connected displays and by the time you see something on screen you have already missed the chance and it starts loading Windows.