Open your continuous form.
Create an AFTERUPDATE event for the CorrectAns field.
CurrentProject.Connection.Execute "UPDATE tablename SET fieldname=0 WHERE quesidfieldname=" & Me.quesidcontrolname & " And answeridfieldname<>" & Me.answeridcontrolname
tablename = the name of the table you are updating (the answers table)
fieldname = the name of the yes/no field in that table
quesidfieldname = the name of the field in the table that contains the QuestionID
questidcontrolname = the name of the (hidden?) control on your form that contains the current QuestionID
answeridfieldname = the ID field that uniquely identifies this answer (for this question) in your table
answercontrolname = the control on your form (likely hidden) that contains the ID of the current answer record