To set the margins correctly first open your document (your dissertation).
Go to the File menu and select Page Setup.
In the Page Setup window the Margins tab should be viewable, if not click the Margins tab.
You should now see the Margins options.
Either type the correct measurement in each box or use the arrows to increase or decrease the size.
Then click OK.
The margin changes will be applied to your document.
Once the margins change you may notice that the layout of your pages changes ie page numbers. You should now check through the document to check that everything looks ok. If you have manually created the table of contents some page numbers may have changed.