when trying to open up worlds already created a notice comes up saying" the world is corrupted can not open"

We purchased the mine craft pocket addition all as been fine for about 8months, today my son opened it up and none of the worlds would open reading " this world is corrupted can not open"

We tried deleting the worlds but would not delete, so uninstalled and installed the app but still read the same notice

I would like to have it fixed and hopefully not have to pay again for the app please

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