I’d like to find out how you can find and replace microcode in a bios rom file?
I have the latest bios made in 2018 for biostar tb250-BTC but I don’t think it is spectre meltdown proof.
I’m not sure what guide to use if any because of the uefi bios.

Here is two guides about updating microcodes
[GUIDE] AMI INTEL CPU Microcode Update Guide
[GUIDE] Update CPU Microcode + Fix FIT Using UEFITool / Hex

Here is tool MC Extractor to check if microcodes are the latest version or not, drag and drop BIOS onto tool, downloads in the "releases" tab
https://github.com/platomav/MCExtractor

Or, you can also use UBU to update the microcodes for you, but it does have some limits (only 2 microcodes after update usually)
[Tool Guide+News] "UEFI BIOS Updater" (UBU)

If you cannot get it done, or would rather someone do it, let me know I can update for you.

*Edit, I checked your BIOS, and only one (506E8) microcode is updated to latest version, three are not (906E9, 506E3, 50654).
Also, once you update microcodes you will need to correct the FIT Table too.