You might be wondering when you see this error because you haven't rooted your android device so far, then how this kind of error will occur and the same thing happened to me when i was trying to update my Samsung galaxy device.
Finally i found a solution with the help of some article around the internet and i hope with just few steps you can easily able to update your Samsung galaxy device.
Disclaimer : Do it on your own risk and we are not responsible for any kind of damage to your device.
1. Un mount your SD card if inserted.
2. Power off your device
3. Go to Android recovery mode
- Tab 2 - Press Volume Up + Power button
- Galaxy S4 - Press Volume down + power button
- Other devices - Please check in the internet that how to switch on your device with android recovery mode
4. Select the "Wipe cache partition".
5. Restart your device after the successful wipe.
6. That's it . Now you can check for update via OTA android update like you do under setting > About > Check for update.
After the above steps, now am not seeing the same error like "Your device has been modified. Software updates are not available" and my device is updated.