There are two ways of getting files onto your site, and once there, changing them.
2. By using an FTP or SFTP client. This guide will show you how to use FileZilla.
FTP or "File Transfer Protocol" has been the most widely used transfer protocol for over thirty years. Use SFTP or "Secure File Transfer Protocol" if your host supports it. This transfers your files and your password over a secured connection, and should therefore be used instead of FTP whenever possible. Sometimes you have to contact your host to have SFTP enabled on your account.
Why use FileZilla? Because, like WordPress, it is released under the GPL. So, it is not just free, it is staying that way, too. FileZilla is also a stable client that works on all flavors of Windows.
Need an FTP or SFTP client for your Mac? Try Cyberduck, which is also free and released under the GPL.
The following pages will show you how to setup and use Filezilla:
Want to try a different FTP or SFTP client? Find more on Wikipedia.