The specs say this PC supports 9th Gen CPUs, so -in theory- even an older BIOS should be OK. If you want to update BIOS, you'll have to do it with the old/working CPU installed. Then unplug PC and swap in the new one. Next, reset BIOS by removing the motherboard battery and pressing/holding the power button for ~30 sec before reinstalling the battery (time for a fresh battery?) to make sure BIOS sees the CPU change.
Dell Inspiron 3670 manufactured 3/2019 BIOS 1.5.9 I tried to install a brand new boxed 9th generation I7 9700 and the computer won't POST.  I tried reseating the CPU but still no POST.  I reinstalled the i3 8100 it came with and it boots normally.   Would a BIOS update to the latest version solve this problem or is there another issue I need to address. Thanks for any and all help.
No boot after cpu upgrade