You can change your Joomla website's design in a few seconds if you wish. Only thing you need is a proper Joomla template with your website's version. After you have it, you should;
1) Click the Extension Manager button
2) Choose your template file and click the Upload & Install button
3) Click the Extensions>Templates buttons and set new template as default.
If you want to learn how to install new theme to Joomla, click the video icon below to watch video tutorial.