Navigate to the Menu and click on Discussions.
Click on the My Discussions tab.
Locate the discussion you would like to edit and click on its title.
Click on the Edit link.
Click on Change Activity.
Choose how to locate the activity you want to assign the discussion to i.e. My Activities or finding it from the Library by clicking on the buttons. Alternatively, you can assign the discussion as a General Discussion (if enabled).
Click on Next.
Search for the activity (if you are assigning the discussion to one) in the popup window. If you have selected My Activities, you will also get a list of activities that you can select from.
Click on the activity's title.
You will be returned to the Edit Discussion popup window and you will see that the assigned activity has changed.
Click on Save.