Can I upgrade my 8gb iphone to a 16 gb?

So I went and got my iPhone 4s in October 2013 from best buy. Before I went I looked at their website and made sure that the 16gb one was free. When I got there the lady working had tried every way possible to talk me out of it but i didn't listen and I'm glad because I love my phone. Finally she gave up and set up my phone and everything but she didn't ask my anything about how i wanted it, like what color what gb do i want and all that, she just handed me the phone so i assumed she gave me the 16gb one but when i got home i realized that after downloading a few apps i was running out of space because she really gave me the 8gb by then it was too late so I've been dealing with it and now i can't stand having to delete everything and I was wondering if there was a way i could get the 16gb without having to go in and pay so much money for it?

Sorry that was so long I just wanted to make sure I explained.

After this long, you may have problems. You need to read the actual contract you signed when you got the phone… If it says you bought a 16Gb model, you may be able to get a manager involved and get what you paid for. If the contract says 8Gb model, you will have to pay for the upgrade no matter how much you complain.