In order to force redirection of your Wordpress website from HTTP to HTTPS, the key task that you will need to do is to modify .htaccess file that is present in the root of the website directory.
If you check your wordpress install, you will have the following code by default in your .htaccess file.
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
# BEGIN WordPress
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>
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