Prior to recalibrating, please delete the controls.cfg file which is located in your Documents/iRacing folder. If you have chosen the option "Use Custom Controls for This Car", you may also need to delete the controls.cfg file located in your Documents/iRacing/setups/%car% folder as well.
You can then recalibrate your controller by launching the SIM (in a Test session for example) and it should then prompt you to recalibrate your controller.
Click Options and make sure you are in the Drive tab
in the Input Calibration section, click either Steering, Pedals or Gearbox - depending on what you want to recalibrate