Although you can't update the auto generated title pages directly within Atticus, you can absolutely customize a title page to fit your brand and style.
You can either:
Replace it with a Full Page Image
Click the Title Page in the left navigation pane
Since the newest update, you'll see a link near the top of that page that says Convert to Full Page Image
This option will give you the most flexibility to insert a full page image (png or jpeg) or a designed image of your book cover. For more information on sizing your title page, you may want to check out this tutorial as it includes calculators to help you size your image for your trim size: https://www.atticus.io/calculate-image-size-for-print-and-ebooks/
Replace it with a standard chapter page
Delete the auto generated title page (click the little x beside the title when you hover over it)
Click add a New Chapter and replace the title and subtitle information with your book or part details.