To change the terrain quality for Fallen Earth, first you need to login via the Fallen Earth Updater. Next click on "Options" located in the bottom left of the updater window. Under Video Settings you can change the terrain quality by clicking on the desired level (low, medium, high or very high). When finished click on "Save". Next time you launch the game the settings will take effect.
You may also change the option from your in-game options window (click Menu, then Options), however it will require a restart of the client for any changes made to be applied.