From your Profile page you can click on an activity to view the details page. From here you can access the Edit and Delete options.
Synced activities
Please note that for synced activities you cannot change the distance or duration. You can, however, modify the type of activity. This is useful when you accidentally selected the wrong activity type on your device or app and want to correct this to make an activity count towards your challenge goal. This option is also available from the Edit page.