Hi,
you have to change the properties of the scenario in which you want to change something. So if it's an existing one it's best to go to the scenario editor from the main menu and select the one you want to change. Your camera will be positioned directly at that marker. Double click on it and on the right side of the screen there is a new panel, maybe you have to move your mouse there to make it visible. It looks like this:

1) is for the weather setting, from sunshine to sick thunderstorms everythings in there, also snow.
2) is for the season setting which changes the look of trees and buildings and if you set it to winter sets snow everywhere and all objects which have snow textures will show these. Should be about 80% of the default assets, but not all freeware objects come with one.
And I am pretty sure this is also covered in the manuals
