Instructions
Select the Digimon you want to revert into an egg and leave your farm.
Enter that Digimon in up to 10 battles without allowing the Digimon to rest or eat in between the battles. Fighting so many battles in a row will tire your Digimon out.
Put your Digimon back into its cage and don't feed or water it for a few days in game time. The Digimon will revert back to an egg form in 2 to 3 days game time. When you see the needle symbol that means the Digimon is sick and will revert to an egg soon.