Acer Iconia A100 Official ROM (ICS 4.0.3)
1-Download Firmware .  
2-Unzip the file using winrar or winzip . You will find update.zip folder inside. 
3 - Copy the update.zip on a micro sd. 
4 - Now enter in recovery mode and choose install from Sd card.  
5 - Select "Yes" and wait for installation start. 
6-Perform wipe concerned or only when the start is going to throw FC's for the launcher and others. 
7 - Using complete / update package:
ROM Name:Official ICS 4.0.3 ROM
Created By:Acer
Device:Acer Iconia A100