To be able to change your equipped chassis part you must first own an Official car. These can be purchased using Coins or real money from the in-game shop.
To change your equipped chassis part, you must first enter your car setup by selecting “Setup” on the main menu. Once you’re in your car setup you can see your currently equipped chassis part on the left of the screen, represented by the image of an F1 car.
By tapping on that you will see all of your available chassis parts. Tap on the one you would like to use and tap “Equip”. Doing so will equip that chassis part and allow you to edit its R&D setup.