For basic inversion of pins, you can right click a pin and choose Invert. An inverter will be placed at this pin when the design is generated. You can also right click a pin and choose Tie Low or Tie High if you want to connect the pin to either GND or VCC.
To tie an input bus to a constant, right click the bus and choose Tie to Constant. To mark an output pin as unused, right click the pin and choose Mark as Unused.
To clear these, just right click on the pin again and choose Clear Attribute.