DO you mean PC start up hangs and does not get to Windows startup ?
or Windows startup Hangs (where)?
What is the error message if any ?
Have you changed anything recently eg different version of Windows or processor/CPU/Graphics card/drivers/Virus?
BIOS update is unlikely to fix anything if no changes were made
BIOS update is unlikely to work if PC can not start up
Recent Dell BIOS updaters start from within Windows, shut down the PC and apply the update
(not clear if before or after reboot)
It was easier when PC's had floppy disks and BIOS update ran from a MS DOS bootable floppy disk