1. Log in to your dashboard
2. Select Appearance > Select A New Theme from the left menu
3. Scroll down to see the themes. (or search for a specific theme by name)
4. To activate a theme, hover over the theme image and click Activate.
Note: You can preview themes before activating to get a general idea of how it will look with your content by clicking Live Preview to open the theme in the Customizer. This will just be a 'general' idea because each theme has different options and layouts so your content may actually look broken in the preview.
The best way to see the theme in action is to go to the theme designer's website to see a fully functioning demo.
Here are the sites that we currently offer and recommend:
What Themes Does BlogPress Offer?