I got my iPhone 6 on Christmas eve.
And yesterday night I found dead pixels on my phone that I was not aware of.
And I check the the Mobile shop return policy, 14 days, 30 air time, 50mb.
But its little bit over 50mb. 53mb i used.
Can I still go and ask exchange for a new phone?
You should be fine but all you can do is try. Keep in mind that it may be difficult to get a replacement device at this time due to the time of year and limited stock of such a popular device so you may have to just return it and wait for more stock to come in, again all you can do is try.
You'll be fine. The usage limits are only for returns, not exchanges. The usage limits are to avoid being sent a bill if you return your phone within 15 days. If you go over the usage and return your phone, you will pay for the usage you used.