Tuning becomes available gradually as you progress through stages 4-5 in Career mode. For example, base gearbox tuning opens up after Stage 4 Race 4, after that when you complete Stage 4 Race 9 - you can start changing specific gears settings. And finally after Stage 5 Race 5 - Engine Speed Limiter becomes available.

You can access tuning screen by going to main menu - Garage - Upgrade - Tune.