See also the official documentation:
Help Center > Group Owners Manual > Managing group topics and messages > Merging topics

In the Topics View, go to one of the threads that you want to merge into the original 

Use the drop down arrow beside the member name and timestamp and select Start Merge.

After the page refreshes, go to the original thread that you want to merge into and select Merge Into from the drop down. 

Repeat for any other threads that need to go into the original.

If you change your mind after using Start Merge, there will be a choice of Cancel Merge on the drop down menu under each topic.

More information on splitting and merging can be found here: https://groups.io/g/GroupManagersForum/message/14090