From Online Manual
You can merge topics into one by following these steps:
- Go to the topic you want to merge. At the bottom of the page, click the "Merge" button.
- Select the location of the second topic in the "Target board" drop-down menu. This will generate a list of topics contained within the target board.
- Locate the correct topic in this list and press the "merge" button in front of the topic title.
- From there, you can change the subject of the fully merged topic to either the subject of one of the merged topics 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". Note that the merged topic will retain the topic ID number of the oldest topic, regardless of which topic is selected first, so any links to newer merged topics will not work.
Topics to be merged also have tick boxes for "Include notifications?". If you decide to remove the check from these boxes notifications for the merged topic will no longer be sent to any members of your forum who had previously subscribed to receive notifications from either of the merged topics.
An alternative way to merge topics is to type the ID of the second topic in the box ID of destination topic and click the merge topics button.
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.