1. do site backup consisting of code directory and a DB dump
2. check that your server will respond to port 443 ... in a browser, https://yoursite:443
Yes, that will error but as long as web service is talking 443 then next step
3. install Let's Encript ... more than likely you'll have to restart web services ... after that do the same check above.
Look at the secure Icon to check the cert.
4. change the config.php file
5. access the site via https:// it will look messed up .... theme won't look normal. login. Navigate to the replace tool you need and run the replace ...
6. purge cache.
8. clear browser of cookies and cache to your site
9. Access site again using https ... and as a test access with http to see how Moodle re-acts.
'spirit of sharing', Ken