How to enable/disable ARC in an xcode project?

If a project has already been created with ARC disabled, how do I enable it and vice versa?

Open your project and select Edit -> Refactor -> Convert to Objective-C ARC. This will start checking your code if it is ready for the conversion.

See also Clang documentation: Objective-C Automatic Reference Counting (ARC)

How to disable ARC has been answered here

Following are the steps which I did and it worked for me

1. Select Project
2. Select Targets
3. From the right panel, select Build Settings
4. Search for "Automatic Reference Counting";
5. Locate Apple LLVM compiler 3.0 - Language and Objective-C Automatic Reference Counting and select NO in all three sections.