Arab Computers Riyadh, Saudi Arabia

I want to complain about apple service center in Riyadh, Saudi Arabia, I bought a new iPhone 6 last year and within one month of using it had some fault ( the phone restarted on its own and hanged very often. the Apple service center in Riyadh under the name of "ARAB COMPUTERS" after keeping my phone for 3 weeks told me that my phone has many issues and they will give me a new iPhone. The manager came to me and said here is your new iPhone, but he did not unpack the phone in front of me, he just gave me the phone without any packing or accessories. he also said the phone has been registered under the same EMEI number. I didn't know about the technical issues he was talking about, and I am sure that he gave me my old phone on the name of a " NEW PHONE". please guide me how to deal with these cheaters and I am sure they do the same thing with many more people who do not know the technicalities. I am posting the link of their website.

http://www.arabcomputers.com.sa/