To edit your Session History, first click the profile icon on the top lefthand corner of the home screen. Next, click "Calendar History." This will bring you to your session history.
You can delete sessions listed there by swiping left on the session and tapping the delete icon that appears.
You can also add new sessions to your Session History by using the 'Add Session' button below the calendar on the Profile screen or completing meditations or sleep stories.
Any sessions you add or delete will be reflected in your total meditation stats and streak.
Note: Music sessions are not currently added to Session History or profile stats.