The "FS" command is used to replace one string with
  another string.  If the specified text is found, it is
  deleted and replaced with the new text, and the pointer
  is positioned at the end of the specified text.  If the
  specified text is not found, the pointer is positioned
  at the beginning of the buffer.

  The "FS" command is of the form:

      FSold-text<ESC>new-text<ESC>

  For example, to replace the next occurrence of "exumple"
  with "example", use the command

      FSexumple$example$

  (Note that the <ESC> key echoes as "$" at your terminal.)