Answer by Dávid Pásztor for Replace Substring with Markdown on NSTextView in Swift

You should use the replaceCharacters(in:with:) method of NSText (whose subclass NSTextView is).

@IBAction func clickBold(_ sender: NSButton) {
  let range = content.selectedRange()
  let selectedText = (content.string as NSString).substring(with: range)
  content.replaceCharacters(in: range, with: "**\(selectedText)**") 
}