How do I change my screenshots in the App Store?

1 Log into iTunes Connect
2 Click on "Manage Apps" and then the app you want to edit. 
3 Click "View Details" on the version of your app. 
4 Then click the "Edit" button next to Metadata and Uploads.
5 You can click the close button (x) to delete a screenshot, and click Choose File to upload a new one.
6 After you have made the changes you want, make sure to click Save. Or click Cancel to exit without saving.

Refer to this documentation from Apple, Table 7-9 for requirements: 

https://developer.apple.com/library/ios/documentation/LanguagesUtilities/Conceptual/iTunesConnect_Guide/8_AddingNewApps/AddingNewApps.html#//apple_ref/doc/uid/TP40011225-CH13-SW42

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