To start accessing your administrator login page via HTTPS you should edit the code of two files in the administrator folder of your Joomla installation.

Edit index.php file in the administrator folder of your Joomla installation.

In administrator/index.php, immediately after the line that says

define( '_VALID_MOS', 1);

Add the following:

//Redirect to https if accessed over http (except when running locally) if ($_SERVER['SERVER_NAME'] != "localhost") 
{ $port = $_SERVER["SERVER_PORT"]; 
$ssl_port = "443";  //Change 443 to whatever port you use for https (443 is the default and will work in most cases) 
if ($port != $ssl_port)
{ 
$host = $_SERVER["HTTP_HOST"]; 
$uri = $_SERVER["REQUEST_URI"];
header("Location: https://$host$uri"); 
} 
}

Edit also index2.php file in the administrator folder of your Joomla installation.
In administrator/index2.php, immediately after the line that says
require_once( '../configuration.php' );
add the same code as above:

//Redirect to https if accessed over http (except when running locally) if ($_SERVER['SERVER_NAME'] != "localhost") 
{ $port = $_SERVER["SERVER_PORT"]; 
$ssl_port = "443";  //Change 443 to whatever port you use for https (443 is the default and will work in most cases) 
if ($port != $ssl_port)
{ 
$host = $_SERVER["HTTP_HOST"]; 
$uri = $_SERVER["REQUEST_URI"];
header("Location: https://$host$uri"); 
} 
}