Disabling back button and put Cancel button in navigation view controller — like contacts iPhone application

I have a navigation view controller and there are 3 view controllers in the navigation stack. Now on the third and top most visible view controller I have a default back button coming in.

I need to bring this view controller in edit mode which I did... Now the requirement is to have a cancel button as the left bar button item instead of back button.

This is similar to the functionality given by contacts application of iPhone where you edit a particular contact.

Any clue how to achieve this?