There are two ways to go about editing a Designed Catalog template.
1. Click on Edit under the Ad Creative to customize a single creative.
3. Select Save and then Publish / Update Catalog once you're done with edits.
(When you want edits to apply to ALL creatives)
1. Click on Master Template and then click Edit under the template variation in use*.
3. Select Save and then Publish / Update catalog.
*For changes to apply to ALL creatives, make sure that you have customized the variations in use.