error installing iOS 9.3 in iPhone 6

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Resolve iOS update and restore errors in iTunes - Apple Support

Get help with iOS update and restore errors - Apple Support

Do what the message says to do. Try again later. The servers might still be very busy. In the meantime you can try restarting your phone and give it another attempt, try another WiFi network, or try using iTunes to update the device.

I have the same message on my phone. Does anyone know or resolved the error issue installing the update? I already did the option connected in computer and into itunes and does not work either.

Have you tried putting the phone into recovery mode?

If you can't update or restore your iPhone, iPad, or iPod touch - Apple Support