Hi Alie,
unfortunately this is not that easy.
You have different masters applied to the document pages.
Masters could govern page size and margins on document pages and also size and position of threaded text frames on document pages. However to make that work best you need the so-called Primary Textframes on the masters.
Things get more complicated if you did overrides on margins values, did changes to the size of pages using the Page tool that override the values on the masters and you did not work with the Primary Textframe feature. Best never do that if you later want to change page size and margins where text frames should follow.
To give you the best options you now have with this document we need to have a look into your document.
Maybe it's even best to start from scratch with the intended new page size and margin values?
Regards,
Uwe