- click on the grading icon for the quiz (yellow ruler with green check mark),
- click on the questions tab,
- click on "Update all attempts" (as of June 2011)
- click on the question that needs to be changed,
- click again on the grading icon on the right (yellow ruler with green check mark).
- Check the box "Give to attempts with the following answer  points" and
- insert 1 or the correct value of the item in the box.
You will get a modification report looking like this:
Date: Sep 14, 2009 9:46 PM
Modified By: Tanya Joosten
Action: Attempts with the answer below received 1 points