Put a case on the phone. You can't exchange your iPhone for a different color once you've purchased it, unless you're still within the return period, which is usually about 14 days. And you probably can't exchange an iPhone that already has cosmetic damage.
Or, sell the black iPhone privately (make sure to turn off Find My iPhone!), and then use the money from the sale to buy a white iPhone.
Even i know this i can sell it and buy another one and also i know hot to use it with a case but, the main problem is that compared to Apple product, there is no product with as much as poor quality as iphone 5. Complately issue with the case !
It is better to change it with another brand, and i will not buy another iphone again.