DX12 is still beta. Anyway with game closed open Usercfg.opt and change “PreferD3D12 0” to “PreferD3D12 1” in the top block of settings. Save changes and restart game.
Changes to this file can´t be done while game is running as game will always override that file after you quit game and will put there the settings that were configured in the options menu.
Cheers