Log into the back-end of your site (www.your-site.com/administrator/)

Click on: Extensions -> Extension Manager

You see the page "Extension Manager"

From here you can install your Templates, Plugins, Modules, Components and Languages. You have three options:

1. Install from Web - (select an extension listed on the Joomla! Extension Directory)
2. Upload Package File - (select a package from your PC, upload and install it)
3. Install from Directory - (enter the path where the package is located on your webserver)
4. Install from URL - (enter the URL to the package)

Option 4 does everything for you, your environment needs to be properly configured to allow this. Option 2 requires you to download the extension to your PC. Option 3 requires you to unzip and move the folders/files to your webserver. All options require the webserver to have write access to the webspace.

Here, we choose the second method: Upload Package File

Select the package from your PC and click the button "Upload File & Install"