First download FTP software, please see https://www.versio.uk/articles/148-what-ftp-software-do-you-recommend. Below you'll find information on how to work with FileZilla.
Open your FTP software and provide you FTP host, user name and password. You have received this information by e-mail. If you lost them, please see https://www.versio.uk/articles/151-i-have-lost-my-hosting-account-details-directadminftp-what-should-i-do.
You are presented with two screens. The left screen is your own computer, the right screen is your webhosting account. Double click "Domains" in the right screen.
In the right screen, double click the domain name to which you wish to upload.
Double click "public_html".
In the left screen, you can navigate to a folder on your own computer. You can update files and folders by dragging them to the right screen.