You need the exact model of your motherboard.
Then you can visit MSI's website and check that the 3900X is supported on that motherboard and what bios update you need.
MSI motherboards should be able to flash the bios without a CPU but you would need another computer to download the files and set up the USB.
I believe all current MSI motherboards (& ASUS) support bios flashback allowing you to flash the motherboard with just the power supply connected but I may be wrong. I'm sure someone will correct me if I'm wrong.