You open the forum app, go to subscribed threads, and read all the unread ones (bold text). Sometimes the unread ones are on the second or third page, if you've missed them for a while. If you don't really care what you haven't read, you can tap on the icon in the top right corner and chose mark all read.
Or maybe not. Mine only shows a refresh icon at the moment. I'll have to keep an eye on it and see if I get the other icon back when I've got some unread subscriptions.
Alternately, if you just want the red badges to go away forever, not matter what you have or haven't read, do this: Go to Settings >> Notifications, find the iPF app, tap on it and turn off Badges. While you are there, look through the other apps for the other notifications that may have annoyed you in the past. Or maybe there is one you want to annoy you in the future.