You can get bios here
That SMBIOS 2.3.1 I think is referring to BIOS type not version because newest version listed on ASRock site is 2.10
If it works with new cpu you can use it to update bios. I think it should be working but it might not recognize it fully and show as unknown CPU and might not work at full speed.
Since You have P4 CPU I would probably use that for BIOS if E5400 is not properly recognised just to be safe.
If it is working and recognizing E5400 then probably there is no need to update.