From Online Manual
Going to the Topic to be merged. Click on the button at the bottom of the Topic that says, "Merge." Select the "Target Board" by clicking the drop-down box and selecting the Board which contains the Topic that the original Topic will be merged with. You'll now see a list of the available topics that you can merge the topic with. Browse through the pages until you find the right topic. Once found, click on the button to the left of the Topic name that says "Merge." From there, you can change the subject of the fully merged Topic to either of the topics that will be merged or you can select your own. This can be done by clicking on the drop-box and editing the content as you wish. Upon selecting a new or pre-existing subject, you can make it so that all posts in the newly merged Topic share the same subject by checking the box next to "Change the subject of all the messages." When you've confirmed the settings you changed, click on the button that says "Merge Topics." From there, you can go to the "Message Index" or to the "Newly Merged Topic."
If quick moderation checkboxes are enabled, you can merge any selected topics on the same Board directly from the message index. To do so, check the boxes next to each Topic you want to merge, then select "Merge Selected" from the drop-down box at the bottom of the page.