You can mark a workout as complete by:
-Click onto the workout overview
-Press the '...' icon
-Press "Mark workout as completed"
This will then mark your workout as completed.
During a workout you can mark it as complete by:
-Clicking pause in the workout
-Click "End Workout"
-Choose a reason from the top as well as "Mark Workout Complete"
For On Demand workouts you won't be able to manually mark these as complete, if you need to complete one of these workouts 'start' the workout and then skip to the end of the workout by using the slider at the bottom of the screen.