Mobile Device Software Problems

We bought a Samsung Mobile from Foschini, a month ago, Immediately noticed the phone was slow and not doing the commands its was intended for example going into gallery takes me to WhatsApp, thought it was lack of memory bought a bigger memory card in vain, took device to Vodacom Campus Square one of the guys there advised software problems, took the mobile back to Foschini to find unprofessional services as well as an error during our purchase, when purchasing the device my husband was asked 6 or 12 months he asked what the difference was and the assistant said non, however back at Foschini for the EXCHANGE we asked again the differences and was advised that one is with interest free needless to say he changed that, that was fine then after lots of verbal exchanges as if we were responsibly for the device malfunctioning, we then had no choice but to agree to the device being fixed, 3 weeks later we get called mobile is ready all excited picked it up just to find same problem just with one more added we put the cover back on in the car and to our surprise cover doesn't fit flush???? I