Head over to Cartman’s place and meet him in the Coon Lair (basement). Sit across him and he’ll allow you to “dual-class”. On top of the other two initial classes you didn’t pick before, you’ll have the option to choose from the following classes: Elementalist, Cyborg, and Psychic. Choosing your secondary class will depend on your original intended playstyle. Blaster+Elementalist and Brutalisk+Cyborg are a few examples of good dual-class combinations since you only need to focus on specific stats to further complement their damage output.
After selecting your second class, you’ll have to go through a revised backstory scene where you’ll get to test your newly acquired powers. Like before, you just need to follow Cartman’s instructions to defeat the intruders.
After the backstory scene, the Powers app will be installed in your phone. The game will give you a brief tutorial about accessing the Power app and changing your equipped powers. You can only have 3 attack powers and one special power. You can do this anytime you want by launching the app again.