Although you can't update the auto generated title pages directly, 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 or a designed image of your book cover
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.