You can only access the tuning options if your Driving Aids are set to Hard or higher. To change your Driving Aids settings, go to Options and select the Driving tab. Adjust your Driving Aids Preset as desired.
If your Driving Aids are set to Hard or Higher, when in the at-track menu, press the button indicated on the bottom of the screen to adjust your tuning. You can either use a slider to make simple adjustments between "Loose" and "Tight", or you can edit a custom setup and fully tweak your car.