MTN K90 East rand

I upgraded my phone on the 18th of November. I am hot happy with the phone. I then went back to the store to return it as it's only four days. The consultant advised that they cannot take the phone back as it's already opened. The store opens the phone to the client when upgrading or activating the new line to check is everything is fine so the client goes home with the opened phone. I only need to choose a different device, I cannot keep the phone for 24 months while I am unhappy with it, It's not fair to me. Cellphone number in question : *** Contact number : ***