How to iterate through the GalleryItems collection and update corresponding records in the 'PO Items' list based on the ID
GalleryItems
ClearCollect( GalleryItems, Glry_Po_Items.AllItems// Replace 'Gallery1' with your gallery control name );
ForAll( GalleryItems, Patch( 'PO Items', LookUp('PO Items', ID = ThisItem.ID), { Title: NewPONumber } ) );