So, caveat first. This is not the suggested approach if you are running a transactional website. If you’re going to be losing money for every second your site isn’t working, you ought to do this as a proper planned and tested project.
But if it doesn’t really matter if things don’t work out, and you know how to revert to a previous version of your site, you can just go for it.
Firstly, take a backup using something like ManageWP, which is really easy to use to revert the site in the event of a disaster.
Then install a Lets Encrypt SSL. This is completely host-specific, but most big hosts will have this option for you. It is usually free. Often it’s a self-service option in CPanel.
Once you’ve done that, log into WordPress, and under settings change the URL of the site from http:// to https://
Now you can either install a plugin (like Search and Replace) or use PHPMyAdmin to do a global search and replace of your old URL to your new one. Code will be something like this:
UPDATE wp_posts SET post_content = REPLACE (
'Item to replace here'
'Replacement text here'