my iphone5 wifi and 3g are dropping and cannot connect to internet or connect for short time then go weak, drop, or no connectivity

i bought my iphone5 from electronics shop, after 2 weaks i couldnt solve the problem of wifi or 3g

so i sent it back to the shop, and they change some parts from the iphone5 and they told me its working now!

i asked for replacement but they refuse, so i refused to recieve my iphone..

i cannot accept opening my iphone5 and replacing parts without approval from me as an owner..

the shop confermed that the iphone5 is defective, and thats the point.

also i couldnt get help from apple cuz i live in kuwait,

the question is: who's responsible about iphone5 factory defective? and can i get a replacement iphone from the shop or apple?

or i must recieve and use my stomache opened brand new iphone 5 ?