Instructions
Keep the Digimon in your party and battle with him to raise his level. Each time you level up, all of your stats, including Aptitude, are increased.
Purchase farm goods that raise Aptitude. You can purchase these from most shops in the game. The items that raise Aptitude are the Digimon Encyclopedia, Miracle Spring and Digimon Lab. You must also buy food for your Digimon to eat while it is training.
Train your Digimon at the Digifarm. To access the Digifarm, go back to the Tamer Home, where your character lives, and interact with the computer on the left side of the desk. When prompted, choose the Digimon whose Aptitude you want to raise and leave it at the Digifarm.
Check in with your Digimon every once in a while. This helps raise your Digimon's Friendship level while it is training. You will also receive reports from the Digifarm as you play the game. Check them to see how your Digimon is progressing. Pick up your Digimon when you are satisfied with its Aptitude level.