MTN Upgrade not wanting to exchange cellphone

So on the 2 May I went into the Hemingway’s store in East London to be greeted by a consultant the manager and the lady kept on telling me they can’t exchange my phone and saying how must we resell it if the box has been opened the consultant opened the box in the first place and secondly there were no phones on display the day I did my upgrade I felt very disappointed in MTN and will be changing my whole family over to a better network MTN has gone down if a customer doesn’t want a phone or has had second thoughts on a phone they must be able to change it like GAME stores they change anything no questions no making the customer feel small I will be fighting this case I’m not paying for a phone i don’t want if I’m not happy there’s a CPA in place to protect us from MTN please could you help me and try contact the management of MTN to please help me I’ve been waiting for a response for two weeks now thank you ***