To add a saved grid template to your story, click on Templates from the media panel on the left hand side of the editor.
With the templates panel open, you can see all of the available template and template categories.
Hover over each template to see the PREVIEW and ADD options.
Clicking on preview will open the template on a larger modal:
Once you've selected the right template, click ADD to enter it into your story.
Keep in mind, Maker made templates use the Pro Layout Builder. Once you add a template in Pro Layout Builder, you cannot convert it into a different type of grid.
Tip: To learn how to bulk replace images in a grid, visit this page.
Helping users with visual guides and technical support.