When the iteration is done (on time), make a demo for the product owner. Nothing but a demo of running software has any real value.
Let the developers demo each their part of the new work, and check each user story for completion. When the product owner is satisfied with all user stories, close the iteration. If a user story is not finished or accepted, move it to the next iteration or to the backlog.
Use this happy occasion to reflect, get feedback and be creative. For every release, you should also have a retrospective, making the team, project, collaboration and dialogue even better.