Unless you have another index file like index.htm or index.php that is taking precedence this is likely just a matter of needing to clear your browser cache. If you connect through a corporate or school network or are on certain ISPs it is also possible that the old version of your page is being cached elsewhere. In general I would recommend clearing the browser cache or trying a different browser first to rule that out.

You will want to edit this file:

sudo nano /etc/apache2/mods-enabled/dir.conf

For example. My website is in PHP, so my dir.conf file looks like this:

<IfModule mod_dir.c>
    DirectoryIndex index.php index.html index.cgi index.pl index.xhtml index.htm
</IfModule>

Since you want index.html to be served first, edit your file like this:

<IfModule mod_dir.c>
    DirectoryIndex index.html index.cgi index.pl index.php index.xhtml index.htm
</IfModule>