Q: while updating i phone 6  using  itunes , it updates with otr update by default

Hello jagannath,

Thanks for using Apple Support Communities.

To update your iOS version in iTunes please follow through the steps below.

Update your device using iTunes

If you can’t update wirelessly, you can update with iTunes. If your computer uses the Personal Hotspot on the iOS device that you’re updating, connect your computer to a different network.

1.  Install the latest version of iTunes on your computer.
2. Connect your device to your computer.
3. Open iTunes and select your device.
4. Click Summary, then click Check for Update. 
   
5. Click Download and Update.

If you see error messages in iTunes or need more space to update

Get help when you see error messages while updating your device with iTunes. Or if you need more space for an update, you can delete content from your device manually.

Update the iOS software on your iPhone, iPad, or iPod touch

Cheers.