Install via Admin Panel (packed template file)
Log in the Back-end (www.your-site.com/administrator/)
Click on: Extensions -> Install/Uninstall
You see the page "Extension Manager"
From here you can install your Templates, Plugins, Modules, Components and Languages. For both you have three options:
For example here we choose the first method - Upload Package File
Select the package from your PC and click the button "Upload File & Install"
When the package contains no errors you are done and get a success message like the one below.
Install via FTP (unpacked template file)
Templates can also be installed via FTP without pack them before. Simply select the template folder(s) on your PC and upload it to your server.
Upload dir must be: /path_to_joomla/templates/
This method is mostly used when you have created a template yourself and do not want extra pack it to install the template or if you want to upload more than one template at once.
Note: Do not do this with Components, Modules and Plugins if you are a beginner/novice, because they need database entries which must set up manually when you use this way.