Will BIOS reset delete files?

Resetting the bios shouldn’t have any effect or damage your computer in any way. All it does is reset everything to it’s default. As for your old CPU being frequency locked to what your old one was, it could be settings, or it could also be a CPU which is not (fully) supported by your current bios.

What happens when reset BIOS?

Resetting your BIOS restores it to the last saved configuration, so the procedure can also be used to revert your system after making other changes. Whatever situation you may be dealing with, remember that resetting your BIOS is a simple procedure for new and experienced users alike.