Can I revert an action item that has been completed by mistake?

No, completed action items cannot be reverted. However, they can be deleted and started anew. If you have completed action items yourself by mistake, you will need to create a new action item - use the same details mentioned in the action item, which was completed by mistake, and then re-approve it as desired. 

When a person is assigned an action item in eCompliance, they can approve/ reject it.

Managers who have access to 'Edit Any Action Item' would be able to approve it and also have access to complete it by themselves. If the action is completed, it will be locked. 

Please ensure that an action item is submitted as "Completed" only as a final step to avoid this and the action appearing in reports.