Instructions
Finish the "Dodonga at Dongara Hills" quest by killing Dodonga. He will drop the Mater Sprout, an item you will need to access Mater, the tree of life.
Return to your village and bring the Mater Sprout to Mater, the large tree in the middle of the village. You will gain access to a new screen which allows you to make your Yumipons evolve into other characters.
Collect Ka-ching by killing enemies and completing quests. Ka-ching is the currency used in "Patapon 2" and you will need it to make your Yumipon evolve.
Return to Mater and click it. Click on Pyopyo. Your Yumipon will now transform into Pyopyo, a rabbit-like Patapon.
Collect additional Ka-ching, go back to Mater and select Buhyokko, the pig-like Patapon.
Return to Mater after collecting enough Ka-ching to select Menyokki, the tree-like Patapon.
Fight additional enemies and complete additional quests to collect more Ka-ching. Return to Mater and select Megapon. Your Menyokki will now evolve into Megapon.