For Word 2007, Word 2010, Word 2013, and Word 2016:
  • In the open document, click on Page Layout in the top menu.
  • Under Page Layout, click on Margins.
  • Select the desired pre-set margins or select Custom to enter values.
  • For more information, please refer to Microsoft support article: Set page margins in Word

For Word 2003 and earlier:

Same settings for the whole document:
  • With the document open, select File, Page Setup from the menu bar
  • In the Apply To area, select Whole Document to apply the settings to the whole document.
Different settings for different pages:
  • Place the cursor at the end of the page prior to the one in which to change the page settings
  • Select File, Page Setup from the menu bar
  • In the Apply To area, select This point forward. (Do not manually insert a page break or Word inserts a blank page.)