(Information and examples taken from the DroidScript documentation)

The RenameFile method renames a file in internal or exernal file storage.

 app.RenameFile( fileName, newFileName );

function OnStart()
{
  var file = "/sdcard/wftest.txt";
  app.WriteFile( file, "Hello" );
  app.RenameFile( file, "/sdcard/wftest2.txt" );
}