At the "Extras" menu, select the "Promotions" option. Then, enter one of the following codes by selecting the corresponding aliens to unlock the corresponding upgrade:
Enter Feedback, Fourarms, Diamondhead, Bloxx as a code to unlock the Gravattack upgrade.Wildvine upgrade
Enter Gravattack, Eye Guy, Arctiguana, Wildmutt as a code to unlock the Wildvine upgrade.XLR8 upgrade
Enter Shocksquatch, Gravattack, Arctiguana, Diamondhead as a code to unlock the XLR8 upgrade.