Instructions
Wait for the first week of May to arrive in the game if you have just started playing. At this time, the Association is unlocked.
Select the Association from the map to travel there. Walk up to the woman behind the desk.
Speak to the woman, and when she asks what you want to do, choose "Retire." When asked if you want to freeze the monster, say "Yes."
Combine the frozen monster with another one, if you wish, to create a new monster with its parents' stats and abilities. If you do not want to combine the monster, you may delete it.