Most gasoline sold in USA now has 10% alcohol content

 I think he's referring to the 30% value that gets stuck in the GMs as seen under fuel trim data

There is a section of the engine module under Live Test that deals with fuel composition reset. In choosing this button, I believe it shows what the present Alcohol content is and there you can go to the reset button. Pressing the button will go to 0 on the chart. Then the data in the scanner does a calculation and from present data. You should see an updated number there. I read that you can really get some solid data by filling up with ethanol free gas. Then do a reset. I have done this procedure with a 2008 flex fuel GM van. It does work well.