The second Preview release of 4.12 is now available to download from the launcher. This release comes with updates and new features for rendering, such as improved high quality reflections, an update for the Dual Coat shader model (to now specify a second normal map for the surface below the clearcoat layer) and support for realtime planar reflections.
To download Preview 2, head over to the Epic Games Launcher and select the "Library" tab. Click "Add Versions", and choose 4.12 Preview 2. If you have a Preview version already installed, simply hit 'Update'.
You can read up on the fixes included in this release through this forum post from Stephen Ellis. Also be sure to have a look at the known issues list to see what's still outstanding. Remember these Preview builds are for testing only, so try not to use them for the active development of your project!