I would make my changes to the template site before uploading it, If you just want to change the text its fairly simple, but if you want to add or change pics then make sure you find the size of the images so that when you change them you dont alter the formatting of the page ( e.g if the picture was larger it would maybe throw all the text underneath it instead of next to it.)
I would usually use Dreamweaver to edit it but there are some free ones like kompozer (
www.kompozer.net) which are very similar to programs like word (wysiwyg editors).
Your web space provider will have the details of how to access your account to upload it, they dont all use dpanel some are manual upload settings which can be done through any modern browser you dont really need any other program just the webspace address, account name and password. The main page a browser looks at is index.htm so this will be the page people look at first so your main page will always be named this.
Hope this helps