Instructions
Battle in every mode possible to raise your rank as a trainer. Train with your Digimon until you reach at least Bronze level. Fight other trainers of Bronze level and above.
Make your Digimon train in the gym to raise its stats. Digimon need their stats raised in order to digivolve.
Allow your Digimon to revert back to an egg. A Digimon has to revert to an egg at least twice before it can digivolve into a MegaDigimon.
Pass time with your Digimon in the game once you are rank Bronze or higher and your Digimon has the proper stats for digivolving. It can digivolve into a MegaDigimon randomly at any time.