-f   Freshen files in a .ZIP file

  When this command is specified, PKZIP will compare the contents of the
  .ZIP file specified on the command line against the files specified for
  compression.  If a file ALREADY EXISTS in the .ZIP file, PKZIP will check
  to see if the file exists in the source area (i.e.  the files specified on
  the command line).  If the file does exist, it then checks to see if the
  file in the source area is more recent than the copy contained in the .ZIP
  file.  If it is more recent, it will replace the compressed file with the
  one found in the source area.

  Using this parameter will normally speed up the ZIP process considerably,
  as PKZIP will be able to skip over files that it has already compressed.

     pkzip -f baka.zip c:\ranma\*.*

  This example checks the contents of the .ZIP file "baka.zip" against the
  files specified by "c:\ranma\*.*".  Any file that matches will be updated
  if the file in the source area is newer than the copy within the .ZIP
  file.