How to change the back button label, react-navigation

You can use headerBackTitle prop to control back button title.

headerBackTitle

Title string used by the back button on iOS, or null to disable label. Defaults to the previous scene's headerTitle

Example

const SomeNavigator = StackNavigator({
   Main: { screen: Main },
   Login: { 
     screen: Login,
     navigationOptions: {
       headerBackTitle: 'some label'
     }
   }
 });

UPDATE as of version 5

As of version 5, it is the option headerBackTitleVisible in screenOptions

Example of usage:

<Stack.Navigator
  screenOptions={{
    headerBackTitleVisible: false
  }}
>
  <Stack.Screen name="route-name" component={ScreenComponent} />
</Stack.Navigator>

if you want only to hide in the single screen you can do this by setting the screenOptions on the screen component see below for example:

<Stack.Screen options={{headerBackTitleVisible: false}} name="route-name" component={ScreenComponent} />