Case against sony

Hi Last year in month of may I purchased Sony xperia c4. But this year just before two days from the expiry of warranty, I took it to the service centre bc of some issues. Service centre people kept it for 7 days and after 7 days they called me back and said I will get a replacement with xperia z5 upgraded mobile. From then I am not able to use it properly. Because of the problems in z5 mobile I hve to submit it number of times to the service centre. Last time when I submit it again they said they will give me a replacement set once again. But they give me a handset which is given back by some other customer because of some problems. Now on last Friday I submitted it again because of the problems in that mobile. Again they told me that they will now give me a fresh mobile. But today I got a call from them now they are saying that I will get a replacement but with some other used mobile. Plz guide me what I should do in this matter. I will be waiting for your reply.