Sprints are released together with all the features that are attached to them. You can read more about changes to those features here. It will also notify all the feature subscribers about new updates via email.
Go to the Sprints section and find a sprint that you want to release. Press the button in the Status column.
Choose the "Done" option from the dropdown list
You will be prompted to confirm the release. Just press the "Confirm" button and it's done!
Read more about how features depend on sprints