The most usual (and recommended ways) are:
1. In the wp-config.php file
Insert into the wp-config.php file the following line:define('FORCE_SSL_ADMIN', true);
It must be inserted somewhere above wp-settings.php location.
2. Modify the Apache virtual host settings (or .htaccess file or the main httpd.conf)
# Force SSL for wp-admin folder and wp-login.php file
RewriteEngine On
RewriteCond %{THE_REQUEST} ^[A-Z]{3,9}\ /(.*)\ HTTP/ [NC]
RewriteCond %{HTTPS} !=on [NC]
RewriteRule ^/?(wp-admin/|wp-login\.php) https://your-site.com%{REQUEST_URI}%{QUERY_STRING} [R=301,QSA,L]
# End Force SSL...
Notice!
For both ways, of course, you must also already have SSL configured on the server and a (virtual) host configured for the secure server before your site will work properly with these constants set to true.
A more comprehensive reference here:
https://codex.wordpress.org/Administration_Over_SSL