If you wish to do this programmatically, it can be done like this:
Objective-C
UIBarButtonItem *backItem = [[UIBarButtonItem alloc] initWithTitle:@"Custom"
style:UIBarButtonItemStyleBordered
target:nil
action:nil];
[self.navigationItem setBackBarButtonItem:backItem];
Swift
let backItem = UIBarButtonItem(title: "Custom", style: .Bordered, target: nil, action: nil)
navigationItem.backBarButtonItem = backItem
However, if you prefer using Interface Builder, just select the UINavigationItem that you wish to set the back button for, and navigate to the attributes inspector to change the back button’s title.
NOTE: It is important to remember that you must configure this on the view controller that you would be returning to upon tapping the back button, not the currently visible view controller.