Some themes you may leave zipped and use the admin area of WordPress to install – some you have to FTP to the themes directory. Here are instructions for both ways:
Zip Files:
1. Log in to the WordPress Administration Panel.
2. Select the Appearance panel, then Themes.
3. Select Install Themes
4. Use the Upload link in the top sub-menu to upload a zipped copy of a Theme that you have previously downloaded to your machine.
5. After it is uploaded, click the activation link.
Theme Folders
1. Use an FTP client such as FileZilla to connect to your hosting account.
2. Navigate to where WordPress is installed and locate the /wp-content/themes/ directory.
3. Upload the theme folder into the directory.
4. Go back to Appearance > Themes > in WP amdin.
5. Find your Theme and activate it (you may want to click “Preview” first.