ALT+P,M (Layout Ribbon, Margins option)
You can then down arrow through a variety of common margin settings to select from OR if you want custom margins, Hit A, which brings up the Page Layout dialog with the Margins Tab selected and open. The various fields in the Margins tab are self explanatory (except, perhaps for Gutter, which I never change).
Brian - Windows 10 Home, 64-Bit, Version 1809, Build 17763
Explanations exist; they have existed for all time; there is always a well-known solution to every human problem — neat, plausible, and wrong.
~ H.L. Mencken, AKA The Sage of Baltimore