First, you need a tool for uploading theme files to your site, you can use filemanager from cPanel or any FTP clients.
Here are?some good ones?:
FileZilla for windows.
Cyberduck for Mac.
Extract theme zipped file, then upload extracted theme folder to?/wp-content/themes/
?directory on your site. so after uploading it , you must have something like?/wp-content/themes/your_theme/