Open Steam, Rightclick on the Fernbus Simulator -> "Manage" -> "Browse local files"
Now the Fernbus Simulator folder should have popped up, navigate to: Fernbus Simulator\Engine\Extras\Redist\en-us
There you should find a installer called "UE4PrereqSetup_x64", run it as administrator and try to start the Fernbus again.

In your Library, you click with the right mouse button on the entry of the "Fernbus Simulator"
In the drop-menu, you click on the entry "Properties"
In the window "Fernbus Simulator - Properties" you select the tab "BETAS"
In the field "Select the beta you would like to opt into:" select the entry "beta"
Now you can close the window and the beta version of the update should steam download right away
After downloading the update you can start the Fernbus Simulator via the "PLAY" button