You can mark a workout as complete by:
-Choosing your workout
-Press "Start Workout"
-Press the ... icon
-Press "Mark workout as completed"
This will then mark your workout as completed.
Whilst you're in a workout you can mark it as complete by:
-Clicking pause in the workout
-Click "End Workout"
-Click "Quit"
-Choose a reason from the top as well as "Mark Workout Complete"