You don't have to do anything special for games that use the Arduboy2 library. It's not you that's selecting what library to use, it's the game itself.
You just have to install the Arduboy2 library the same way that the Arduboy library is instaled:
By using the Arduino IDE's Library Manager. You do that by following Step 2 in the Quick Start Guide or by following Mike's instructions above.
Once you've done that, you don't have to touch anything else.
A game will then automatically find and use Arduboy2 if that's what it has been programmed to do.