Choosing a product
You can link any product you have in your store to a landing page. In other words, all the items you see in your WordPress admin area → Products → All items are eligible to be featured on a landing page. When choosing a product, make sure it is published so that your visitors can see it.
Content seen on the demo landing pages can (and should) be replaced with your own copy, visuals, etc. It is used only for illustrative purposes so you have a better understanding how landing pages might look and how elements can be arranged on a page.
Also, you're free to create a few landing pages for the same product using different templates. You can even create several landing pages for one product using the same template and style them in a different manner.
To sum it up, you can use any product you like and it doesn't have to have anything to do with the demo content.
Changing a product
You can always link another product to your landing page. To do so, go to your landing page settings and click the Trash icon in the Landing Page Product area.
To select a new product, click the Add Product button.
In the pop-up window, hit the + button to link another product to your current landing page.
Now your landing pages features another item. Save your settings to finish.