Head to the "Journal" tab of your Dashboard and find the workout that needs to be updated. Below the workout's title, click the "edit workout" link.
You will automatically be taken to the workout logger. Locate the workout's date at the top of the logger and click the down arrow to the right of the date.
A calendar will then appear on the screen. Choose the date that you want to move the workout to.
After you select the new date for the workout, click "Log Workout" at the bottom of the screen, and your workout now be logged on the correct date.