Log into the Builder
Click Design and in the dropdown menu click Onboarding Screens
Locate the Splash Screen Image or Onboarding Background Screen and upload your new image. Learn more about the design specifications here
Click Save
Important note:
Your splash screen or onboarding background screen image is a change that requires a new app update in order to go live onto your app. Learn more here about changes that can go live instantly versus changes that need an app update.