Hello,
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Generally, if you want to change the screenshots, you just need to change the URI for the Item you want to change. A possible reason is that the URL is incorrect. You could double-check it.
Should I need to delete the previous screenshots by editing directly in Partner Center before import listing?
This should work.
Another way is that you could delete these two fields when you are importing the store listing, then these two items will not change. After that you could change the screenshots in the Partner Center manually.
Thank you.
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