Log in to your Smart Stories dashboard.

Locate the group of stories you want to delete and click on the group to open it.
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Click on "Edit Story" on the top right of your screen.

Click on "Templates" in the left menu. Look for the pen icon on the right side of the template name and click on it.
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Adjust the top, bottom, left and right padding accordingly and click on "save" at the bottom of the page for the change to be effective.