You will see forums claiming "HP BIOS Unlocker" tools that generate an Smc.bin file on a USB drive. For the G6, the old B1200/B1400 generators do not work. HP changed the SHA-1 algorithm.
For modern business laptops like the HP EliteBook 840 G6 Go to product viewer dialog for this item. hp elitebook 840 g6 bios password reset work
Many local repair shops charge $50–$100 for EliteBook 840 G6 BIOS unlock. You will see forums claiming "HP BIOS Unlocker"