If your forum is embedded my guide at:
http://greghelp.991552.n3.nabble.com/Embedded-Forums-and-https-tp4012846.htmlwill help.
You only need to leave out the bits that refer specifically to an embedded forum, if you simply want your forum to use https.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.