To return to the help topic that was displayed just
  before the one that is currently in the window, click
  on the Go Back button. This feature is useful if you
  want to review something, or you find that you have
  taken the wrong path and you are not getting the help
  you thought you were going to get.

  The Go Back button is dimmed if the current help topic
  is the first topic displayed in the Help window after
  you chose an item on the Using Help menu.