You do have a valid Certificate from a CA (they issue certs) ... and it's installed ... correct?
Site can respond to https:// ... and the lock in the URL line of the browser shows green or a broken icon?
Check your config.php file for the variable wwwroot ... which is a url. Does it have http://site or https://site?
The two ... what's in DB and what's in config.php .., need to match.
That tool is supposed to be part of core now ... not an addon.
There may not be a link in the admin menus for it so ....
Go to: http://yoursite/admin/tool/httpsreplace/
If it is there, strongly suggest making a backup of your DB before using.
If it's not there, how was the Moodle installed?
'spirit of sharing', Ken