For WordPress to work properly on HTTPS, there are three steps.

1. Certificate and redirect

Make sure the domain has an SSL certificate and turn on Force HTTPS Redirect: see cPanel: SSL certificate and redirecting to HTTPS.

2. WordPress addresses

1. Log in to WordPress admin (https://yourdomain.com/wp-admin).
2. Go to Settings → General.
3. In WordPress Address (URL) and Site Address (URL), change http:// to https://.
4. Click Save Changes. You'll need to log in again.

If you manage the site from WP Toolkit, you can change the address there too.

3. Old http links

If some images or files still load over http://, the browser shows the page as not fully secure. See My website shows as not secure.

Note: if a WordPress plugin does its own HTTPS redirect and you also turn on Force HTTPS Redirect, they may conflict. Keep only one of them on.