You can change your default page by editing your .htaccess file. This can be found by going to your cPanel's File Manager and enabling hidden files (or adding &showhidden=1 to the end of your normal URL). Make sure you are editing the .htaccess file in your public_html folder. If you cannot find a .htaccess file, feel free to create a new one.
Add the following code to your .htaccess file:
DirectoryIndex index.php
Make sure nothing else is on the same line as the code above.
I believe that the default index settings is already configured for index.htm, index.html and index.php.
Therefore, you simply need to rename the index.html file and upload an index.php file.