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You need to return your device to the place of purchase or contact Xperia care http://www.sonymobile.com/global-en/support/contact-us/
contact Xperia care or get the brand new z3 screen
If you are in the UK then you can't cancel a phone contract based on the phone - This is in the terms and conditions however what you can say is the phone is or try and say the device is inherently faulty and push for a pro-rata refund or a reduced cancellation fee