Firmware Over-the-Air (FOTA)

I am trying to implement FOTA .... Initially i am trying to send my code from Serialport3. Application receiving the code after that i am using eval.. (function )

function Update(serial) {
  var code = {line:''};
  serial.on('data', function(data) {
    code.line += data;
    var idx = code.line.indexOf('\n');
    while (idx>=0) {    var line = code.line.substr(0, idx);
      code.line = code.line.substr(idx+1);
      print ('\r\n code.line:'+line);
      eval(line);
      idx = code.line.indexOf('\n'); }  }); 
}
function onInit(){
  Serial3.setup(115200);
  Serial3.println("Data on uart3");
  Update(Serial3);  
}
onInit();
save();

```
I am trying this code ..
but here i am getting old and new application from the flash on power on... how we can overwrite old application with new one..?

Thanks and Regards
Vishnu