On the sleep log page, https://www.fitbit.com/sleep you should be able to click either edit or delete in the "Sleep" section. On the upper right corner above the graph, or each graph if there is more than one, you should see an edit symbol (paper with pen) and an X when you hover the mouse over the graph. Click the X and that item disappears. If it doesn't, open a case with Fitbit support.
However, since you only want to change PM to AM, click on edit and make the changes to start and stop times.
