You need to go to www.sports-tracker.com and log in.

Choose a workout. On the top right corner you can find "EDIT".

On the editing page you can find "EXPORT" and save the GPX file. 

 

To Import Workouts
On the dashboard on Latest workouts on top right corner you find "+ADD WORKOUT" and on the next page "IMPORT GPX" 

You can import 10 workout files at once. Supported file type is GPX. If your GPX file is very big or the workout in it is very long the import might not work correctly.

Unfortunately there is no way to merge accounts.  

But you can export workouts from the other account and import them to the other account.