When you are reading a book, briefly press the screen in the MIDDLE (not the edge). A menu will appear across the bottom. One of the entries is "Aa text". Press it and the text menu comes up. You can change the font size, font, margins, and line spacing. Click the "x" in the upper right corner to leave.
This is pretty basic stuff. You should read the Quick-start guide and the User's Guide. To do so, press the physical "n" button at the bottom of the Nook, choose "library" and you should find them there.