Take care, any updating of the bios is fraught with danger and you could end up with a non functioning system.
That said, I would have thought that if (and it might be a big "if")previous BIOS versions were available, the MSI website would be the place to find them.
Whether you will be able to revert is another matter altogether. I am not an expert, but I do know that my motherboard has a dual Bios, meaning that I could, if I wanted to, have two versions of the Bios available. However, without this feature I think that you are faced with flashing the BIOS again with the version that you want, and you might find that you cannot overwrite a newer version with an older version.
Hopefully someone with more knowledge than me will see this and ride to your rescue.