Hello Alex. Well you can go down two separate roads with your question...
that is if you did not download a "payware" route, or a route that the author had made so it cannot be "edited". (Textures and track cannot be changed in "Session".)
OPTION 1: McGuirel has made many seasonal textures that actually change w/ the "Environmental" settings tab in "QuickDrive". Her textures are used by many great route builders such as Msgsapper (and myself). But you will have to check out the many different texture yourself. Make yourself a pallet of all McGuirel's textures; then do a save. Go into "Driver" and the "pallet" you had created. Then go to Environmental settings to "flip" through the seasons; with each flip you will see which textures(s) would work best for you on that route. You will then go to the "Main Menu" tab, and "Bulk Asset Update/Replace" in TaNE ("Replace Assets" in all other versions of Trainz). In the left column you simply put the texture(s) you what to change, for
one of Mcquirel's textures. [You will do this for every change of texture(s).]
As for the track -as far as I know- Tume is the only person who has created "seasonal track". He created it for his "Milwaukee Road Avery-Drexel" route
[the payware route that comes with the "Deluxe" TaNE version] in which case unfortunately, I see you have the "Standard" version! (Should you be interested), you'll need to purchase the "Deluxe Route" package from "SimulatorCentral" in order to obtain Tume's route/track. You would do the same for his track, as had with the textures. [McGuirel had made "seasonal" track (but only a particular track, for a particular season), therefore would not change via the Environmental tab.]
OPTION 2: Now if you do not wish to change the textures and track to the route you downloaded; you'll need to make a copy of the route...
for reasons aforementioned above. Go to the "Main Menu" in that route and select "Save As". Place after the routes name something like "_Winter". (However, if the route has an included session, you will have to go into Session to do this. Then change within this "Winter" copy of that route, the texture(s) and track(s) you desire.
...
Happy "Winter" RR'ing!!! 
