Method 1: To upload the theme via WordPress admin area you need to use the installable zip file. Go to Appearance > Themes > Install Theme and click Upload. Select the yourtheme.zip file on your hard disk.
Please note! If you download the archive named YourTheme_All_Files.zip, you need to unzip it and find yourtheme.zip file inside.
Method 2: If the method above fails you will need to upload via FTP. Please, install Filezilla, unzip the theme and upload /yourtheme/ folder into /your-website.com/wp-content/themes/ directory on your server.