svoid replaceFileZip2Zip(File inZip, File outZip,
  S toReplace, byte[] newContents) {
  
  Zip2Zip z2z = new(inZip, outZip);
  z2z.copyFileContents = (name, in, out) -> {
    if (eq(name, toReplace)) {
      print("REPLACING " + name);
      ctex {
        out.write(newContents);
      }
    } else
      copyStream(in, out);
  };
  z2z.run();
}

Began life as a copy of #1033499