Tutorials›Installing WordPress Themes
Download the theme package ZIP file and extract it (if you're not sure how to do this, check out this tutorial). This will create a folder containing another ZIP file with a name similar to theme-WPNNNNNNN.zip, which is the actual theme ZIP file.
Sign in to your WordPress installation's Dashboard and go to Appearance > Themes > Install Themes.
Click the file upload box, select the theme ZIP file (theme-WPNNNNNNN.zip), and click Install Now.
Once the theme has been uploaded and installed, click Activate and it'll become your blog's active theme. You can also activate the theme later by going to Appearance > Themes and clicking the theme's Activate link.