Dicussions may be rearranged to change the order in which forums and topics are listed.
Select Discussions.

Select the Discussions tool from the Tool Menu of your site.
Select Organize.

Select Organize from the Actions Menu.
Select the drop-down menu.

The numeric dropdown menu allows for reordering the Forum, Topic, or both.
Choose a number.

Choose the appropriate number for the order in which the items should appear. Note that lower-numbered items will appear at the top.
Select Save.

After reorganizing forums and topics in the desired order, select Save.