Firstly, you’ll need to upgrade your vehicle’s turbo. You can do this in the ‘PARTS’ menu, where you can choose between ‘STOCK’ ‘STREET’ ‘RACING’ or ‘PRO. With the turbo upgraded, you can now adjust the turbo pressure using ‘TURBO PSI -/+’ in the ‘TUNING’ menu. The higher the PSI, the higher your car’s turbo pressure.
Once in the game, your car will automatically utilize the turbo pressure applied in the ‘TUNING’ menu. You don’t need to activate it.