Valve's MOBA behemoth has had the Source 2 option for a while, just not in the main client. To use it you download the Reborn update (you'll find it via the Play tab) and then select Reborn beta when you launch the game. But now Reborn is coming to the game's main client and bringing all its accompanying bells and whistles as well as a few works-in-progress.
Talking about the switchover on their blog, Valve's Dota 2 team say:
"Moving away from the Source 1 engine soon will allow us to better focus our efforts entirely on Source 2 in preparation for various upcoming tournaments. There are still a few features that will need some work when the update is released, but we plan to finish them quickly. Expect the weeks that follow to focus on ironing out issues, refining the dashboard and iterating based on your feedback. Afterwards, we’ll begin shifting our attention toward the in-game UI, where we plan to redesign the HUD and work on a variety of other in-game features."
There's no exact timeline for the switch but if you've downloaded the Reborn update already you won't need to do it again when it comes to the main client, so Valve are advising players start doing so now.
*I'm saying this because it's true, rather than as hyperbole - Reborn not only brings aesthetic changes and new features, it also pegs Dota 2 as a game development platform as well as a single game. Designers can use the provided toolset to create their own custom games and then make them available to the playerbase via the Custom Game tab. Basically, Dota will not be provided as a single game anymore.