building a user-friendly site is obviously the least of your worries at this point....
get an FTP program (on a Mac, fetch is my favorite; on a pc try filezilla, you can download it for free)
it's basically a program where you have direct access to files on the server once you enter all the login information.
another option is to use dreamweaver - it has build-in ftp functionality. you set up the site in the site menu and can then upload files as you work on them for testing or whatever.
finally, another option is to just upload/download files straight from the hosting company. for example if they host with godaddy you will have control panel access when you login at the website (godaddy or whatever) and you can look at all the files there. This isn't the best way by any means, but it is *a* way.
I personally prefer FTP although Dreamweaver does have it's pro's as well, it's more convenient if you're actually building the site in Dreamweaver because you don't have to leave the program.
oh yes, like the person above me said, download the files you want to change, make the changes, and then re-upload them to the server.
good luck!