Depends, some motherboards allow a CPU-less and RAM-less BIOS update. This is a very rare feature usually only offered by high end motherboards.
Most motherboards require a CPU in the CPU support list. Before a BIOS update may be performed. In order to add other CPU models to its support list. For instance most 100 series Intel motherboards need a Skylake CPU installed before you can perform a BIOS update to add Kaby Lake support.