See also the official documentation:

• For group owners and moderators: Help Center > Group Owners Manual > Managing group topics and messages > Editing the content of messages in the archive
• For group members: Help Center > Working with group messages > Editing your own messages in the group's archive

You must have permissions to do this - e.g. be an owner or moderator.

Go to Messages on your group home page.

In Messages, change to Topic View using the dropdown menu.

An arrow with a dropdown menu appears just on the bottom right on the signature and date line.

There are links to these functions:

• Close Topic - this function closes a specific thread. In Settings, you can set up a default date for threads to close, but this link allows you to close the specific topic.
• Edit Subject - includes adding a tag
• Make Sticky - this topic would always appear first on your group site. You can also "unstick" a post.
• Start Merge - to merge topics or threads - see Merge instructions
• Delete Topic