Failed to reverse the upgrade

On 10.12.2015 i did the upgrade at Mtn shop in Rustenburg at Impala hospital. The phone(I-phone S6) worked for a day only. Monday morning 14.12.2015 i went back to the shop and they said they are going to replace the phone as it was within 7 days of purchase.<br> I requested not to recieve the same model phone as it dissapointed me. Tuesday 15.12.2015 i returned the contract papers and phone original box as requested by the sales Rep and i told him that if they wont allow me to replace the phone with another model they can reverse the upgarde. On 17.12.2015 i went to confirm at the shop about my reversal, also requested something written to proof that i reverse the upgrade and i was told there is nothing to be given and they have logged on the request for my reversal and they will give them the respond within 72hrs even call Mtn customer service for co operates but unfortunalety my matter was not resolved as i was told by Thulisile Dube (from cooperates deprt) that i am talking a story.