• Go to the Control Panel
• Go to Assessment Manager
• Click the Modify button to the right of the assessment.
• When it loads the test, locate the questions that you wish to change.
• Click the Modify button to the right of the question.

Important Note:  This can only be done when the test is not available.  If you have made the quiz available and students have taken it, you'll lose the scores recorded in the grade book.