Yes, you can reopen a completed sprint anytime from the completed sprints section in Reports. Click the sprint that you want to reopen and select Reopen in the sprint details page. Once you reopen the sprint, it will be directly moved to the board. If you want to extend the duration of the sprint then you should replan the sprint. After you replan the sprint, it will be moved to the Sprint Backlog. Now, you can edit the sprint and update its duration.