How Can I Switch to HTTPS URL After Installing to HTTP?

You have to navigate to application/config/app-config.php and change the constant APP_BASE_URL from starting from http to https.

If you have already configured cron_job with http:// starting url, make sure that you update the cron job url to start with https://

In case the .htaccess code below do not work on your server you need to consult with your server administrator to adjust the .htaccess code.

If you want to force it you will need to add a suitable htaccess code to fit for your needs.
You can try this one and should work well:

RewriteEngine On
RewriteCond %{HTTPS} !=on
RewriteRule ^ https://%{HTTP_HOST}%{REQUEST_URI} [L,R=301]