Swim Trainer gives you the possibility to manually enter your own swim logs or edit existing ones. Swims you swam with your smartwatch are automatically synced with the app (instructions for Garmin here and for Fitbit here).
If you want to add a new log manually, you can do so using the plus buttons at the bottom right of the dashboard or on the „My Swims“ page. If you want to edit an existing swim, select the „edit“ option in the menu of the View Swim Result page.
You can give your swim a title, edit the date and set the pool size.
You can learn how to add or edit individual sets here.