Step 1: Make a copy of your document and work with the copy because some of what follows cannot be undone

Step 2: Clear all section breaks (margin and column settings, header/footer changes, orientation changes)
a) Ctrl-H (Replace)
b) Find: ^b
c) Replace with nothing
d) Replace all
e) Close the replace dialog


Step 3: Select All (Ctrl-A)

Step 4: Ctrl-Spacebar (clear character formatting)

Step 5: Apply the normal style or body-text style

Step 6: click in your text so you don't have everything selected

Note: I am not using a Mac - the Ctrl-A and Ctrl-H may be Cmd-A and Cmd-H

Basic Formatting in Microsoft Word
Sections / Headers and Footers in Microsoft Word 2007-2013

For future reference it is very rare to need to change margins in text; instead, change indents.