On Windows: Save files are stored in your %AppData% folder on your computer. That way you can easily update the game versions on the same computer without needing to carefully transfer anything. You can get there by typing %AppData% into the address bar of your file browser.
Copy and paste this into your Windows Explorer window address bar on both computers after running the game once on the new system: %appdata%\RenPy
That will navigate straight to the save location for all Renpy games. You can copy folders straight across or just do the whole renpy folder if you'd like.