To install a Magento Theme, Sometimes you might face technical difficulties regarding installation.
Anyhow there are several methods of Magento Installation.
FTP ( File Transfer Protocol)
Magento Web Admin / Magento Connect Manager
For installing via FTP You just need to extract your theme first then upload it in the Theme Folder
For installing via Cpanel you need to go to the File Manager. Then Locate the Magento From Directory. Then upload the Theme File in the Theme Folder ( Extracted File )
For installing via Web Admin / Magento Connect Manager select your favorite theme from Magento Connect & click on Install Now
Agree to the terms and get the Extension Key
Click the Select Key so that it could be copied
Now go to your Magento Admin Panel to Login.
System > Magento Connect >Magento Connect Manager
Paste the Extension Key and Click Install
Now go to System > Configuration and then from Left Menu Select Design > Themes
Mention the Name of Theme in Default Theme ( It will set as default ) and click on Save Config
Enjoy Your Theme !
For more Information you can visit : https://www.siteground.com/tutorials/magento/magento_themes.htm