To show more months in the Date Navigator, drag the split bar between it and the calendar.
You can show more months in the Date Navigator by dragging the split bar between the Date Navigator and the main Outlook window. The number of months you'll be able to see when you drag will depend on various factors such as your screen size and resolution.
Be aware that you'll have to choose between whether you want to have more months displayed in the Date Navigator or more detail displayed in the calendar. For example, if the calendar is smaller because you're showing many months in the Date Navigator, the calendar heading may show Mon, Aug 09. Show fewer months in the Date Navigator, and you'll see Monday, August 9 in the calendar heading.