You cannot modify the access settings of a published assessment, but you can remove a published assessment, modify the access settings in the pending assessment, and publish it again. To remove a published assessment, see Retracting or removing an assessment. To change the access settings, see Assessment settings.
You can modify the settings, parts, and questions of any assessment listed under "Pending Assessments". When you publish an assessment, a copy is created and listed under "Published Assessments". For the published copy, you can make changes to all the settings except "Assessment Released To".
Note: After you've published an assessment, if you modify its "Pending Assessments" copy, you'll have to alter the assessment's name before publishing it. When you publish it, you'll be creating another copy under "Published Assessments", not replacing the existing published copy.