I just made a backup of my TSW2 profile since we are probably getting the UE4.26 update tomorrow. I recommend you do the same, just in case something does not go as expected. Hopefully everything will be fine and you will not need to use it, but better be safe than sorry.
Yeah, it is in C:\Users\YOUR_ACCOUNT_NAME\Documents\My Games\TrainSimWorld2. Substitute your YOUR_ACCOUNT_NAME for your actual account name. Make sure TSW2 is NOT running when you do the copy. What I do is just copy the folder with a description of what it is: Example: TrainSimWorld2 => TrainSimWorld2 (last copy before UE4.26)
Another way of doing the same is just to go to your "My Documents" folder. You should find the "My Games" folder there, and the TrainSimWorld2 folder inside of it.