To rollover a futures contract:

1. Within the NinjaTrader Control Center window, select the menu Tools-->Instrument Manager
2. Select the expired contract on the left list of instruments and press the ">" button
3. In the master list of instruments, search for the instrument you wish to rollover and select it
4. In the lower portion of the Instrument Manager, ensure the correct exchange is selected, then select the new expriration date (For Example: 09-06 for September 2006) and press the "<" button to add it back to the list