This forum is intended for cPanel-related issues. Uploading files to File Manager would be a cPanel question. Questions on how to code for webpages on your site would be outside that area.
The best course of action for these questions would be either to contact your hosting provider or post to an online forum devoted to webpage creation.
As for your question to get you in the right direction, you'll need to save your page as either index.html or index.htm if this is a replacement for your existing site's main page. Since you seem to be new at webpage development, I would highly suggest making a backup of your existing file prior to making any changes so you can revert if you happen to make a mistake.