OK, I think I understand your question now.
You need to get an HTML editor. (It's possible to use an ordinary text editor, like Notepad, but something that highlights the code for you makes it easier to read.) There are various free ones, such as [this, [URL="http://notepad2.com/"]this or [URL="http://bluefish.openoffice.nl/index.html"]this. Using that, you edit a copy of your file on your own computer and then upload the file to your site using an FTP client, such as [URL="http://filezilla-project.org/"]Filezilla or [URL="http://www.coffeecup.com/free-ftp/"]FreeFTP](http://www.coffeecup.com/free-editor/). Editing the pages directly on the server is not recommended.
If you just want to change your existing site a little, you should be able to download the files to your computer, make your changes and then upload them again. However, sites built with any kind of site-building software generally have more convoluted code than you want or is necessary. You might find it easier just to start from scratch and build your pages as you want them.