Paste the following code at the top of the .htaccess file to configure your desired index page.
In the example below, we decided to make the index page of our folders named first.html.
#Alternate default index page DirectoryIndex first.html
You can also list more than one file in the configuration.
The file will be read left to right and check for them in that order.
In this example, we add index.htm, index.html, and index.php to the list.
#Alternate default index pages DirectoryIndex first.html index.htm index.html index.php
First the server will check for first.html, if it does not find a file with that name, it continues to index.htm and so on.