Where to have Iphone 6 exchanged for a new one?

I bought my Iphone 6 in T-mobile and currently my Iphone 6 is having this popping creaking sound when you touch the upper right screen. I read similar cases and they went to an Apple store and they exchange it for a new one? Where should I go T-Mobile or Apple store since I bought it from T-Mobile?

T-Mobile.

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