iOS 64 bit support is implemented with the scripting backend called IL2CPP. It runs with your C#/UnityScript code.
The IL2CPP runtime combines an ahead of time compiler with a virtual machine to convert assemblies to C++ while leveraging standard platform C++ compilers to produce native binaries. The net result is significantly higher performance, platform compatibility and maintainability.
IL2CPP is the only scripting backend to support deploying to ARM 64-bit on iOS, and is thus mandatory to deploy to the Apple app-store for releasing new apps.
Pick it in Scripting Backend dropdown in Player Settings.
By default it will build for Universal architecture (including both ARM64 and ARMv7), if needed you might switch to specific architecture in Player Settings. There is number of things that should be done before your application is up and running in 64 bits:
Method not found: 'Default constructor not found...ctor() of System.ComponentModel.Int64Converter'.