There are a few different ways to make a global change in this way. It's hard to know which one to suggest, because we don't know how you set up your document. I'm guessing that you have one frame per page, right? And that frame is sitting directly on the margin guides. (Those are the, um, pink-and-purple guidelines on every page; they might be invisible if you're working in View -> Screen Mode -> Preview, and they might be invisible because your frame is hiding them! You might have to move a text frame out of the way temporarily to see what I'm talking about.) I'm also guessing that all these pages have the same Master Page applied. If so, then:
a) Go to Layout -> Layout Adjustment and turn it on
b) Go to the master page that is applied to your pages (probably A-Master)
c) When you're in the master page, make changes to margins in Layout -> Margins and Columns
That will work, but only if most or all of my assumptions are correct. Some caveats:
1) When making changes to margins in master pages, you might have to shift-select to get both left and right pages of the spread
2) You might not know what I'm talking about w/r/t margin guides...
3) You might not be working in View -> Screen Mode -> Normal, so your margin guides might be invisible
4) You might not have ever heard of a Master Page - that's what I'm guessing, anyways. It's the way to make changes to margins (among other things..) only once and have them apply to many pages.
If every object on your page is snapped to a guide, then when you have Layout Adjustment on, you can change the size of the margin (or even the size of the page!) and your layout will automagically adjust to fit. However, it doesn't work if the objects are not snapped to guides, or very close to guides.
At this point I suspect you'll probably have to do it manually. Layout Adjustment is only effective if your frames are snapped to guides, which yours do not appear to be, and it seems further complicated by the use of two different master pages.
IF and only IF you have used master text frames, adjusting the size and shape of the master frame will be reflected in pages based on that master, BUT ONLY IF you haven't already tried to adjust them on the document page. As soon as you do anything to an overridden master object (and putting text into a master text frame overrides it automatically, whatever attribute you change is no longer linked to the master page.
I allowed Layout Adjustment then it works.