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

In Messages, change to Topic View.

An arrow with a drop down 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.
  
• Edit Subject - includes adding a tag
• Make Sticky - this topic would always appear first on your group site. 
• Start Merge - to merge topics or threads - see Merge instructions
  
• Delete Topic