unhappy with product

I was due for an upgrade on my contract,which was a s4 samsung,my intention was to upgrade to the s6 samsung.when I went to the store I was made to believe that the huewai gmate phone was superior,I upgraded to the hauewai,only to find out it was to big and far less superior to the s6.the next morning I phone customer support,they instructed that I take phone back to switch to s6.on the 8may 2015 I went back to store explaining my problem.only to get a bluntant response from the manager and staff.he insisted the phone cannot be changed.I explained to him that it was not even 24 hours,what about the cooling of period and further more the sales consulant not once told me the phone I took cannot be changed conclusion I was mislead into taking wrong phone.