iPhone 5 displaying single 1x icon instead of 2x?
The app probably doesn't have a 2x icon. Apple doesn't allow apps in the store with only 1x, so I assume this app has not been updated in a very long time?

Contact the developer and ask them to fix it. Nothing else you can do, only the developer of an app can modify it (this is a security policy enforced on iOS to prevent malware attacks).