An exam must be set to Draft Mode before questions can be added. Once you have released or closed an exam you can no longer edit the questions it contains.
If you want to edit the sections and/or questions of an exam you have already created you can do so by going into the Exams tool, clicking into the exam, and then clicking the Manage Questions button.
If the section has been set to randomly select a number of questions then Extra Credit questions cannot be added to it because some students may be given the EC question and some may not.
You can also filter questions by clicking the question type, category, or tag icons from an individual question as well.
Once you have used a question in two or more courses you can no longer edit that question.
Again, you can only edit questions within an exam that is still in draft mode. Once you have released the exam the questions can be viewed but not edited.