After recording, finishing, and syncing an activity, you can still edit it from the mobile app.
First, open the activity by tapping the activity you wish to edit from your Feed. Then tap the Menu button - the three dots - at the top-right of the activity page (highlighted below) and select Edit Activity. Please note that if your phone has a built-in Menu button, you will not see the three dots; you should use your phone's Menu button instead.
You will then be taken to the Edit Activity page, from which you can add an activity title, change the activity type, add a description, change your Gear, or make the activity Private. Tap Save Activity to save any changes you make, or tap Discard or go 'back' to discard your changes.
If you wish to Delete your activity, follow the same steps as above, but select Delete Activity after tapping the Menu button.