Always update BIOS before you update the CPU. So put the old one back and update BIOS. Then try the new CPU. And I don't promise the latest BIOS will support your new CPU. Dell doesn't usually say which / any new CPUs may be supported by a BIOS update.
Keep in mind there's always a small chance a BIOS update will brick the motherboard. So be sure to follow all installation instructions for the flash update. Disconnect all peripherals except mouse, monitor and keyboard before running the update, and you probably should disable your antiviral / antimalware suite before doing the flash too (remember to reactivate the suite after the flash!)