I could not edit value of cookies.
How can I edit my cookies in Safari Web Inspector?
Ask Different is a question and answer site for power users of Apple hardware and software. It only takes a minute to sign up.Sign up to join this community
please reference Safari Web Inspector Guide
Unfortunately you can't edit cookies in the default browser safari in Mac. Try using third-party plugins like safaricookiecutter or use firefox web browser and use firebug.