This is a how to for previewing the new maps before they are 100% released to public for anyone who cares.
In Steam, right click on Killing Floor 2 and select Properties
Click on the Betas tab
In the drop down, select "preview -"
Close the box.
That is it! If done correctly Steam will download the preview data and in the games list will show as "Killing Floor 2 [preview]". I believe this prevents you from playing on non-preview updated servers though / with people on regular version.