Nokia
Mobile not running properly

I have purchased nokia mobile phone from nokia care.after one month i saw my mobile is not running properly.i went to nokia care centre.they said it is a software problem.i said u pl change my nokia phone or give my money back.i said why i accept that kind of mobile which is not working properly with in one month.but he said there is no problem after repair, but after 15 days problem is occured.i request u pl change my cell phone or give my money back.if nokia phone create problem with in one month so why people spent more money for nokia phone