How to enable/disable ARC in an xcode project?

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

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

https://i.stack.imgur.com/UATIJ.png

When you migrate a project to use ARC, the -fobjc-arc compiler flag is set as the default for all Objective-C source files. You can disable ARC for a specific class using the -fno-objc-arc compiler flag for that class. In Xcode, in the target Build Phases tab, open the Compile Sources group to reveal the source file list. Double-click the file for which you want to set the flag, enter -fno-objc-arc in the pop-up panel, then click Done.

In Xcode 5.0.2, select your project in Navigatior, select Build Settings, search for Apple LLVM 5.0 - Language - Objective C and change Objective-C Automatic Refence Counting to NO