Cell C refusing to change my contract device

I did an upgrade from the BB Z10 to the pre owned iPhone 5s with Cell C on the 9th of October 2015. And I was told I have 7 working days to change the device if I have don't want the device. I called today which is the 7th day after I received the device and spoke to a lady called Sharon and asked her that I want to change the device to an iPhone 6s. With an attitude she tells me that Cell C cant take the device back as this Saturday and Sunday are also counted in the 7th days that were referred to in the terms and conditions. As a 2 years loyal Cell C customer I'm very angry at this as now I'm stuck with a phone that I do not want and I'm told that there is nothing that Cell C can do about it. I don't want this phone anymore and if Cell C doesn't want to upgrade me to the iphone 6s I would like to cancel the whole contract and go back to the network I was using. Such treatment is really unfair! I have always paid my contract on time and this is the Thank you I get from Cell C?