FSB speed

Depends on the motherboard in question.

For many older motherboards, FSB is NOT in BIOS settings. It is handled by jumpers on the motherboard.

Some FIC motherboards require the use of a utility program to change CPU settings.

For others, it is on a screen labeled CPU setup.

You should be able to download the manual from the manufacture. They normally show you where all the setup screens are.

Chas

there are hundreds, if not thousands of motherboards, and motherboard models.

you have to be more specific, or
RTFM
Read The F'ing Manual