From Online Manual
Go to the thread that will be moved and select the button that says Move Topic. Upon selecting the button, youll see a page with several options and several different places and ways to move your topic. If you'd like to Post a redirection topic, check the box that says Post a redirection topic.
A redirection Topic includes (by default) the original name of the thread along with a link to the Board it was moved to and a link to the newly moved topic. By default, redirection topics are locked. If you'd like to change the name of the thread that is being moved, check the box next to Change the topic's subject. You'll then see a field where you can enter a new subject. Doing that, you can also opt to make all of the post subjects match that of the new subject by checking the box next to Change every message's subject. Now all you have left to do is select the Board to move the Topic to and then click on the button that says Move Topic.
Mass Moving Topics
If you run into a case where there are a bunch of topics you would like to move out of a board you can mass move topics from one board to another board, by going to the message index of a board and using quick-moderation.
Before you're able to move multiple topics at the same time, you'll have to enable checkboxes. Follow the directions below to enable checkboxes.
Profile > Look and Layout Preferences
To the right of Show quick-moderation on Message index as. You'll see a drop-down box. Click on that box and select the option that says checkboxes.
Now go enter a board and you will see checkboxes to the right of every topic. Check the boxes for the topics you wish to move and then go down to the bottom of the page where you see a dropdown. Select Move Selected To and in the next dropdown select the board you wish to move the topics to.