Phone with faulty battery

I upgraded my contract on the 16th of June and the phone that I got(Samsung) A3 had a faulty battery. The battery does not last the whole day. My wife went back to the shop after 3 days to lay a complaint and was told that there are some applications running in the background and were disabled but the IT/Tech guy said it wouldn't help but the assistant manager insisted that my wife to take it home and assess the situation. I went back to the shop after 6 days and was told the same thing that my wife was told. My wife went back to the shop today and demanded a new phone but was told that she will get a new phone next week Tuesday provided the phone is still in good condition which I don't understand why a brand new phone with a glass screen and a pouch would be in bad state after 7 days. She will be off line for 5 days and this is due to the incompetence of the staff