From the manual (Maps and Navigation section):
"Home and Work locations also display under the map's search bar. Touch to set an address to the corresponding location. After entering the address, touch Save as Home or Save as Work. Then simply touch these shortcuts whenever you want to navigate home or to work.
To change or delete the address associated with Home or Work, press and hold its icon and in the popup screen that displays, enter a new address and touch Save as Home or Save as Work, or touch Clear Homeor Clear Work."