Select the "Promotions" option at the "Extras" menu. Then enter one of the following codes by selecting the corresponding alien.

Gravattack upgrade
Enter Feedback, Fourarms, Diamondhead, Bloxx as a code.

Wildvine upgrade
Enter Gravattack, Eye Guy, Arctiguana, Wildmutt as a code.

XLR8 upgrade
Enter Shocksquatch, Gravattack, Arctiguana, Diamondhead as a code.