The BIOS is provided in a form of memory chip (EEPROM) that does not lose its contents when the power is turned off. A BIOS can either in a ROM chip or a flash memory (EEPROM).
ROM based BIOS is programmed at the factory. It can't be changed without replacing the chip.
Flash-memory BIOS can be updated using a special program provided by the mfg.
"Flashing The Bios"
