To update your app screenshots on the Apple App Store:

1. Go to iTunes Connect, and log in to your iTunes Connect account. 
    
2. Click My Apps.
3. Click on the app you would like to update.
   
4. Click New Version.
5. Enter your app's Version Number. See How do I find my app's version number for Apple? for information on finding your version number.
   
   
6. Click Create.
   
7. Update your screenshots. 
   Note: Images can't contain alpha channels or transparencies. Either save your images as .jpg or update your .png images and delete the transparent areas.
   
   
    
8. Enter a description in the What's New in this Version text box.
   
    
9. Click Edit next to rating to update your apps rating, and then click Done.
   
   Submit your app’s binary file via a Mac PC. See How do I upload my app using the Application Loader? for more information and next steps. 
   Note: This process takes about 10 minutes.
   
   
10. Click + by Build and upload your updated build.
    ​This is only available after your binary file has been uploaded. 
    
     
11. Select the relevant build.
    
    
12. Click Done.
    
     
13. Click Submit for Review.
    
    
14. Select the following:
    
    a) No in Export Compliance
    
    b) No in Content Rights
    
    c) Yes in Advertising Identifier
    
    d) Select Attribute this app installation to a previously served advertisement.
    
    e) Accept the license agreement.
    
    f) Click Submit.