How can i upload my theme using FTP?

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/