There are two ways to access the templates in the app. Click Slides at the top, then Templates. Alternatively, click Media, then Pro Plus Templates.
Use the search bar in the top right to filter, or choose from the gallery. Once selected, click Create Presentation on the template's pop-up window to use it. Presenter will then have you choose a background to start building your slides with.
To change a slide's background, use the Background dropdown menu. To add new slides, click New. You can choose a different background and add your own text to each new slide.
Customize to your liking and click Save when you're finished.
If you'd like to design your own Slide templates, check out this article.