It appears that there are two different ways to do this. If you want it to mark every forum as read, then click on the link that I have highlighted in yellow in the picture below. That link is at the very bottom of the forum.
Now if you want to do this for each individual forum, then you can click on the white circles to the left of the forum. The picture below shows what I am referring to, and shows what it will say when you put the cursor on the white circles.
I'm going to move this over to the support forum since it belongs there. There may be some other ways to do this that I have not mentioned.