Refusal to change an upgrade

I upgraded 15 Dec 2015. I received the phone. The new phone was charged and looked at in order to find out how it works and what the functions are for. The next day it was clear there was a problem with the battery, I returned the phone explaining the problem, on Thursday 17 Dec. I then said that I wish to cancel the upgrade and do a new upgrade with a different phone., as I had lost faith in the first phone. The store refused to do this. After complaining to the consultant at the store I was told to contact the Head Office. This I did, after holding on at the switchboard for at least 5 minutes, I was then put through to a call centre agent, who did not resolve my query, after this I spoke to several consultants being pushed from pillar to post and repeating my story over and over. Eventually a call centre agent did assist me, who told me that she confirmed with a person at retentions that I could do this. I have this confirmed in writing. The agent has now backtracked on her story and now says I can't do this. I'm saying that I cannot be told one thing one day and something different the next day. The managers refuse to deal with me and I cannot resolve this issue.