You can merge topics into one by following these steps:

1. Go to the topic to be merged.
2. Click on the button at the bottom of the topic that says "Merge".
3. Select the "Target board" by clicking the drop-down box and selecting the board which contains the topic with which this topic will be merged. You will now see a list of the available topics with which you can merge this topic.
4. Browse until you find the right topic.
5. Once found, click on the button to the left of the topic name that says "Merge."
6. 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 a new subject. To change the subject, click on the drop-down box and edit the content as you wish. Upon selecting a new or pre-existing subject, you can make all posts in the newly merged topic share the same subject by checking the box next to "Change the subject of all the messages." Once you have made all the alterations, 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.