The cover is simply the first page of a notebook, so you simply change the paper template for the first page.
- Open the notebook and make sure you’re viewing the first page
- Tap the ... button in the upper right
- Tap "Change Template."
- Tap the "Cover" tab, choose a size and orientation from the dropdown lists, and select a cover.
- Tap Apply.
Alternatively, you can move or add another page to the first position of the notebook.