Installing additional templates to your Joomla! is quite straightforward, all you have to do is:
- Download the template zip file.
- Log into your Joomla Administrative panel.
- Click the Extensions tab.
- Select the Extension Manager.
- You will see a button “Choose File” click it and select the template’s Zip file from your computer.
- Hit “Upload and Install“
That is it, your new template should be installed successfully.
*Note: you have to use a template for the Joomla version you are using. For example if you are using Joomla 2.5, you must look for a template that is supported by your Joomla 2.5 version.
Once you have the new template installed you can check more details on how to make it primary here.