A large numbers of processors are "clock locked" - ie are locked internally inside the processor. These cannot be changed from the BIOS itself.
If you fit a new processor, assuming the BIOS can detect it correctly, then it will automatically change it's settings to cope with the new processor.
500MHz celeron systems can normally only be upgraded within certain constraints as to processor type. If you give us more details about the motherboard and new processor, some more detailed help may be possible.