In all probability, you can't, or at least not easily.
The "DELL" image shown on boot is part of the bios code which is read from EEPROM or Flash memory which also holds the basic manufacturers instructions for interfacing with the hardware. Changing this data is a job for people who know what they are doing and who have the source code to the exact version of the BIOS your hardware is using. A tiny mistake here, and you can very easily "Brick" your PC. Permanently.
If you google for
remove dell image from bios[
^] you
may find some instructions, but I suspect you will mostly find warnings like this that it is a very poor idea.
With absolute certainty, even trying to do this would void any remaining warranty you might have!
Seriously. Don't do it. If you don't want the Dell logo, don't buy a Dell!