I don't know how or if it can be done from the USB, I am at work and cannot remember which site I got my update.zip file from. Maybe I can post it this evening when I get home.
To do the reset
From ACER for the factory reset: Acer Support: Frequently Asked Questions list for Internal Only - How do I perform a Factory Data Reset if my Acer ICONIA TAB A500 does not boot?
But there are also many videos, like: Iconia Tab A500 Series - Hard Reset when the screen is black - YouTube