Login to the WordPress site as Administrator, Navigate to Appearance > Themes > Click “Add New”
Evaluate and decide on a Theme you’d like to have installed.
Press “Install”, to install a chosen Theme
Installing Theme: Suffusion 4.4.7
Downloading install package from http://wordpress.org/themes/download/suffusion.4.4.7.zip…
Unpacking the package…
Installing the theme…
Successfully installed the theme Suffusion 4.4.7.
Now, goto Themes and there you’ll find the new theme listed for you to have activated on the blog.