It is possible to return a service request from completed status to open in Service requests > Manage requests
Use the search features to find the service request.
Open the service request to a wider view to view processing history and various actions.
To return the request from completed back to new, click Return to phase New
It is possible to perform various actions on a request in Completed mode without returning it back to open. Under the Additional functions button, you find the actions that can be performed for an already completed request. 