How to change the default directory index page using an .htaccess file?

You can use the DirectoryIndex directive in an .htaccess file 
to specify a custom file or files that the web server looks for 
when a visitor requests a directory. To enable the DirectoryIndex 
directive, use a text editor to modify the .htaccess file as 
follows. Replace filename with the file that you want to 
display whenever a user requests the directory:

DirectoryIndex filename

You can also specify multiple filenames, and the web server
 will search for each file until it finds a match. Consider 
this example directive:

DirectoryIndex index.php index.html index.htm