Ensure you are using a high-quality USB cable and that no other print jobs are in the queue.
Run the .exe (Windows) or .dmg (Mac) firmware file you downloaded. Select your HP 8620 from the list of devices. hp officejet pro 8620 firmware downgrade better
: Downgrading is not officially supported by HP and carries a risk of "bricking" (permanently disabling) the printer. Ensure you are using a high-quality USB cable
Here is a comprehensive guide on why downgrading is better and how to do it safely. Why Downgrade Your HP 8620 Firmware? hp officejet pro 8620 firmware downgrade better