Depending on the BIOS type, usually before the POST beep, you should hit F1, F2 or DEL key a few times until it pops up into the BIOS. Then most BIOS vendors will let you configure the floppy on the first system tab, or the boot tab.
What kind of PC do you have? Dell, HP or so? They usually display a logo on startup... and that will be the time you want to try tapping F1, F2 or DEL key.