Well, here's how Eleven got out of the Upside Down in Stranger Things

Introducing Endless Mode: A New Games & Anime Site from Paste
We’re only a couple weeks away from Stranger Things’ return, and thus far Netflix has teased it with a handful of trailers that show the stakes considerably raised. There are bigger action setpieces, a much bigger monster, and Steve’s hair now stands at absolutely vertigo-inducing heights, a Lovecraftian portal into the macabre balanced atop his head. But one of the trailer’s repeated holy-shit moments is the return of Eleven, now with a new haircut. The character disappeared at the end of the first season along with the monster, but they’ve made no bones about her reappearance in the new season.