On the game manager, winner scene, you put the name of another scene
On your" MainScene" there is an object called GameHandler or GameManager if you select it and watch the details panel it says win game load scene "WinScene", lose game load scene "LoseScene", you just need to put there the name of your secondary scene. for example if you have a scene called "SecondaryScene" put that. Rember to add the scene to the build