There should be detailed instructions included with the flash software. Follow those TO THE LETTER.
Also remember that you will need the exactly correct version of the upgrade file. Just the correct chipset or card version isn't enough, most flash updates also are for a particular version of the currently installed BIOS.
Most refusals of BIOS upgrades to work are due to trying to use an incompatible upgrade package.
If the flash does start, do not interrupt it for anything! If you do your card will likely be useless and only the manufacturer will maybe be able to revive it.
Best thing to do with BIOS: leave it alone.
Unless there's a critical flaw in it that requires an upgrade to fix there's almost never a good enough reason to take the risk of loosing your machine (or in this case videocard). The only reason I ever flashed a BIOS was to fix a Y2K bug in a mobo for example.