You have to upload your files to the root folder called public_html. Inside this folder replace the file named index.html with the desired homepage for your domain and website. You can use any supported file extension as long as you name it index, for example, index.html, index.htm for HTML files or index.php for PHP programming.
If your file is called index, it will be the default file loaded in that folder, and since public_html is your web root folder, the index file inside will be your homepage or main page your visitors will see when they browse to your domain or website.
If you still see the default web page, check that your browser cache is not storing a temporary version on your PC. Clean your cache or browser temporary files and check again.
Note: IndeX.htm is not the same as index.htm
The server makes differences between case sensitive so those would be two different files. Always use lowercase for your web files.