To switch themes, from the dashboard navigation bar, select appearance\themes. This will bring you to the themes overview page where you can change a theme.
A theme controls the general look and feel of your site including things like page layout, widget locations, and default font and color choices. WordPress.com has a wide range of themes for you to choose from in the Theme Showcase where each theme has an about page that includes features and instructions. New themes are always being added, so click on the Trending view in the Theme Showcase to see what’s new and popular. You can change your theme as often as you like.