how use ftp?

You have to use some kind of FTP client (filezila is one of the best in my opinion)

then you need:
ftp url (most of the times starts with ftp.*, something like ftp.mydomain.com)
username (find it in control panel or use same as hosting)
pass (in some cases you need to create ftp account pass in user panel)


then simply connect to ftp using filkzilla and navigate inner directories to public_html (this one is main dir of your website)