There are a number of different ways to check if you're using an Online Store 2.0 theme. The easiest way to check is to enter your theme editor, and navigate to a Product page. If you see an option to add a section to the product page, then you're using an Online Store 2.0 theme.
On vintage themes, you can only add new sections to the homepage, so checking to see if you can add a section to a product page is the quickest way to tell if you're using an Online Store 2.0 theme. If you're still unsure, we recommend contacting your theme developer.
Once you know what kind of theme you're using, you can use whichever install instructions apply to your theme: