There are some steps you need to follow in order to install the template package.

1. Login to the backend of your site.
2. Go to Extensions → Extensions Manager, click on Browse button, locate the .zip file of the template you have downloaded and click on Upload & Install button.
3. After the success message go to Extensions → Template Manager, locate the template you just installed and edit it. Check all parameters and fill in where needed. For more details on that, advise each template's documentation or the Quickstart package you have downloaded.
4. Once done with that, Save your changes by clicking on the Save button.
5. As a last step, make this template the Default one. To achive that, choose it from the list and select the Make Default button from the menu on the top of the screen.

Note: If you are using the Quickstart package always update the template package as it may contain changes.