Themes define the look and feel of your WordPress site. Changing a WordPress theme doesn’t change the content of the site. You pages and articles remain intact. It’s the design that changes. The following take you through choosing a theme.
Step  − Click on Appearance → Themes as shown in the following screenshot.
Step  − Click on Add New.
Step  − Hover your mouse pointer over a desired theme and click Install.
Step  − Click on Activate.
You can now visit your site to view the changes.