adding boxes


Question: I have already inserted boxes within the 'Main' section of a page now I find need to add another box as the first box under 'Main'.... I can't seem to find a way to get the added box to be within the Main box if not added initially. I'm sure I'm missing something here.... on the attached screenshot I'm trying to get the box labeled 'shop-page' to nest inside the 'Main' box not after it. Thanks!!  

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