After you have finished a sprint and are looking to move things forward, you can select Complete at the top of any individual sprint in the Sprints view or select Complete at the top right corner of the Sprint Progress view as shown below.
After you select complete, there will be a pop-up to give you a summary of the sprint. You can see how much effort was completed and how many pull requests were actually closed (via Github integration). Finally, you can select which sprint you would want any remaining tasks to go to in the drop-down at the bottom of the pop-up and select Complete.
If you have enabled Slack integration, upon the completion of a sprint all members will receive a quick report on Slack with information on the completed effort, tasks, PRs, and the total number of commits.