Teacher created assessments in Assessment Builder can be edited up until the point it is assigned to students. Once assigned, assessment questions cannot be re-ordered.
If you wish to edit an assessment that has been assigned, you will need to make a copy of the assessment. You can then edit and assign to students, once ready. Note: You may want to un-assign the initial assessment to avoid confusion with students.
Note: Only assessments created by educators can be edited, DE assessments can not be edited and therefore the questions can not be reordered.