Both aren't technically wrong, but they just sound a little strange switched.
"Settle down" can mean what the commenter above me mentioned, but also there's a term that some people use: "They/He/She settled down" which basically means somebody got married or maybe moved to one place and lived there for awhile, while "settling in" can mean someone that is finally adjusting to a new life after moving to somewhere new.
Just a few more meanings of those terms and a little more about how they differ, I hope this helped!