Reset back factory bios after updating
If you're working on a computer with a Phoenix BIOS that has Main - Advanced - Security listed at the top of the screen, you can get to this setting by using the right arrow to move over to Advanced.
Under Advanced, arrow down to the Reset Configuration Data option and change the value from Disabled or No to Enabled or Yes.
I had the same issue I updated my bios from 0503 to 0802 and my system was looping BIOS IS UPDATING and never finished.
Do not shut down or reset the system to prevent system bootup failure.
The Q-Code is 90 So far it's over an hour and still waiting.
They are an Apple authorized service provider with locations in over 20 cities.
This wiki How teaches you how to update the BIOS software for a Windows computer.