Apologies, I have no idea why that response was empty.
You can edit a sprint's status by simply clicking on Settings when you are viewing the sprint, which will take you to a Sprint settings view, which will in turn allow you to un-mark it as complete.
What was the answer to this in the end? Not sure if it's accidentally been hidden or answered somewhere else.
Ed, response above now replaced, hopefully that answers your question.
Customer support service by UserEcho