In order to delete a sprint, select Manage All Sprints from Sprints drop down menu at the top of the screen.
From the result screen, click the check box for all Sprints that you want to delete. Once Sprints are selected, press the trash can icon on the top. Once a Sprint has been deleted, all Issues contained in the sprint will be disassociated from the sprint and will be listed in the Issue Tracker. Similarly, User Stories will go to the backlog.