New to LibreOffice. This is probably a very simple question. I use the Writer component of LibreOffice, on a Mac running OS X.11.02 (El Capitan).
In menu press Format - Page - then tab Page – and on the left bottom you can change the margins.
Menu | Format | Page edits the margins for the current page style.
Margins are specific to each page style, so for a consistent look you will have to set margins for both the Left Page and Right Page, as well as any other page styles you have defined.