Use the Margins gallery to quickly change the margins for your document.
If you’re not already in Edit view, click Edit Document > Edit in Word Online.
Click Page Layout > Margins.
The Margins gallery appears. The Normal margin, which is selected by default, sets one inch margins on each side of the page.
To select another page margin, click any of the other options in the Margins gallery. If none of the preset margins are what you want, choose Custom Margins to create custom margins.