If a kid is being hyper and driving you crazy, you can tell him, "Settle down!" This is a command to relax and you'd also say this to a dog that's jumping all over you. If a golfer is making bad puts because of nerves, his or her coach might say, "Settle down more when you're standing over a put." When you've moved somewhere new and become comfortable, you could say, "I'm all settled in and I love the new apartment."

Settled down refers to a change in lifestyle. For instance if you had a rogue man traveling the world finally getting a home and a family then he is settling down. Meaning his lifestyle has become more stable and he now tied down to this home. Settling in refers to a move after they have altered the place to their liking and have grown accustomed to the new place. So it sounds like Settling in would be your best bet.