Changing Apple ID on iPhone 5
From the Apple Support Site


   Use these steps to reset your Apple ID:
  
  
  Sign out of iCloud, the iTunes Store, App Store, FaceTime, Find My Friends, Find My iPhone, and iMessage on each device that uses your current Apple ID for these services.
  Go to My Apple ID.
  Select Manage your Apple ID and sign in. Forgot your password?
  Select Edit next to Apple ID and Primary Email Address.
  Enter the email address that you want to use as your Apple ID, then select Save Changes. Apple will send a verification email to that address.
  Open the email from Apple, then click Verify Now. Didn't receive the email?
  When the My Apple ID page opens, sign in with your new Apple ID email address and password. When you see a message that verification is complete, you can start using your updated Apple ID.
  Update the features and services that you use with Apple ID, so that each one is using your updated Apple ID.
  
  
  If you need to change the Apple ID that's signed in to your iOS device, use these steps to sign out, then back in with a different Apple ID.