Go to your FTP program and open up a file called .htaccess
Sometimes this file can be hidden so be absolutely sure that no files are hidden from showing up.
Go ahead and open up .htaccess and add in “DirectoryIndex index.php” without the quotes. Assuming you didn’t rename the default index.php, then this should do the trick. If you’ve renamed the default index.php file then use “DirectoryIndex customName.php”