2 months old new Apple Iphone S6s Upgrade

I upgaraded my account on the 5th November 2015, in the 3rd week of January my phone was not charging as it should hours would go by and not mch battery life added to charge. On the 8th of Feb I charged my phone from morning till next morning would not charge past 3%, I took it into Cresta Vodacom repairs spoke to a very helpful gentleman called Carlo, he advised that as it was practically brand new someone would look it over and replace the device. I received a sms saying my phone is ready for collection, I left it a day then went into store to be told no they have had to upgrade my software and it was charging 100%, I looked at phone and actually it was 24%, I then asked the lady who helped me as Carlo was on his day off (would have rather have dealt with Carlo) to please charge my phone in store 15 minutes went by and not a damn increase. Then a technician came out and said let me take it to the back for 20 minutes and see what happens.REALLY, so I let it happen. he then came out and oh yes its now charging increased by 5 %.I have had to go back today as same thing, to be phoned and said again its charged 100% I want a brand new phone its still in WARRANTY! Faulty I Pay for 24M