A theme controls how your site looks.

1. Log in to your WordPress dashboard.
2. Go to Appearance > Themes and click Add New.
3. Browse or search the theme directory, or click Upload Theme to install a .zip you've bought.
4. Click Install, then Activate.

Tip: Preview a theme before activating. If you've customised your current theme heavily, use a child theme so updates don't overwrite your changes.