If you are limited to WordPress admin area but having FTP access or you want to upload theme via FTP then this can be a method to upload theme. But its not standard method. You can use it if you are not willing to add theme using WP admin area.
- After downloading theme you should unzip it to your computer.
- Using FileZilla or Such FTP client you may access your host web server. Just input your FTP information and then connect to your server and browse appropriate project directory and then go to wp-content > themes and here you need to drop your theme file and wait until its done completely. After successful transfer of the theme you will find it at your theme list from WordPress admin panel Appearance > Themes.