Prior to the release of Deus Ex: Mankind Divided, the team at Eidos Montreal told players that support for the DirectX 12 API wouldn’t be available at launch, but it would arrive sometime on the week of September 5. The developers stayed true to their promise; a new patch for the game is now available, which includes the option to switch to DirectX 12.
In order to activate the new API for the game, you’ll need to opt-in into the “dx12_preview” patch in the Betas tab of the game’s Steam properties page. This is because the studio is still putting the final touches on DirectX 12, and it needs community feedback in order to make the last minute adjustments before the finalized version is released on the week of September 19.
If you have a multi-GPU setup for your PC, note that this version of DirectX 12 won’t support it just yet; you’ll have to wait until the final release if you want to take full advantage of your hardware. There’s also a known bug that might cause some high-end GPUs to "regress relative" to DirectX 11. However, the developers are working on a fix.
The DirectX 12 addition is part of the game’s fifth patch, which addresses multiple gameplay issues, Steam Cloud saves, and UI improvements. If you want the full patch notes, you can find them on the game’s Steam news page.