How to change parts?

All parts - unlocked and locked - you can find in the dart section, where you can change your sets. On the very top, you have your three sets. The green border around the set means which dart is active, and every change you make will be on that set. Of course, you can tap on the other sets to choose them.

Then you have a list of equipped parts.

If you want to change the part, go down to where you have all your available parts. Tap on the part that you're interested in, and you will get a pop-up window where you can compare those two parts and choose. If you decide to change part, tap the green button "Select".