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:
Option 3 does everything for you, your environment needs to be properly configured to allow this. Option 1 requires you to download the extension to your PC. Option 2 requires you to unzip and move the folders/files to your webserver. All option require the webserver to have write access to the webspace.
Here, we choose the first method: Upload Package File
Select the package from your PC and click the button "Upload File & Install"
If the package contains no errors you are done and get a success message like the one below.
Go from Install to Manage, set the type filter to 'template'. The installed template should appear in the list.
If it does, the template is properly installed. Now you can assign one of styles to a menuitem or set the style as default.
For administrator templates make sure you have two windows of the template manager. Activate the new template in one of them and reset it in the other one if something went wrong.
Templates can also be installed via FTP without packaging them. Simply select the template folder on your PC and upload it to your server, using your favorite FTP software. Be sure you upload the template folder to the directory: /path_to_joomla/templates/ - where /path_to_joomla/ is the location of your Joomla! installation on the server. This method is mostly used when you have created a template yourself, and do not want to have to package it to install the template, or if you want to upload more than one template at once. You must then go into extension manager and click on Discover in the sub menu. Click on "Discover" in the toolbar if your template doesn't immediately appear. A list of uninstalled items in the FTP should then appear. Click the checkbox to the left of your template and click install.
Note: Do not try to use FTP to install Components, Modules and Plugins if you are a beginner/novice. These items need database entries which have to be inserted manually if you bypass the package installer.
Joomla Version 3.0 and above can import content along with templates. This is a means to develop content locally with programs like Artisteer so you can import your whole site in one go. To import your content; Install the template as above, then from the Template Manager: Styles page (Extension > Template Manager) Select your template name in the left hand column (with the heading style), Click the Options tab and find 'Import Content from Template' Button. Click the import button which will activate a pop up modal window then click import or cancel.