You mean your existing data?
Start Trainz.
At the Launcher, click on Trainz settings.
Click on the Install tab.
Click on the ... on the right.
A Windows Explorer window opens up...
Browse to where your old data folder is located.
Once you find it, click the select button.
Trainz will need to be restarted after a quick database translations message.
If you can't find your old data folder, then it's probably hidden in the AppData folder. You need to show hidden folders.
You could even move your data folder to another location and rename it to something else prior to performing the steps above. If you have another hard drive, I highly recommend putting the data folder in that location and then connecting to it following the steps above.