HP will tell you that downgrading violates your end-user license agreement (EULA). They argue that security updates protect against remote exploits.
If the update stalls, restart both the computer and the printer, then try a different USB port. hp officejet pro 8620 firmware downgrade better
The HP Officejet Pro 8620 Firmware Downgrade: Taking Back Your Printer HP will tell you that downgrading violates your