Thanks for using Bitnami. There is no central location for that. Each php application you deploy over the stack will have its own database configuration, which you can point to your RDS instance.
In the case of
phpMyAdmin the configuration is in
/opt/bitnami/apps/phpmyadmin/htdocs/config.inc.php. Look for the line
$cfg['Servers'][$i]['host'] = 'localhost'
To change the database host, and there are lines below to change the port and other settings.
If you are not going to use the bundled MySQL database, you may want to stop it, using
sudo /opt/bitnami/ctlscript.sh stop mysql