You'll have to either take it to a shop and ask them to flash it with a cpu it does support or you'll have to buy a cpu it supports. Then flash it then put this CPU in it
The only way youre going to flash the BIOS if the mobo doesnt support it, is if its an ASUS mobo that supports USB BIOS flashback
Since you can flash a BIOS with just a PSU
Other brands dont have this feature. And other brands, the system has to be running to flash a BIOS