Well, you've defined the variable as
export meaning it will be visible for you to set a value in the Inspector, so that's what you need to do.
In the Inspector, you can assign the scene you want in a couple of ways:
1) Drag the tscn file from the FileSystem tab and drop it on the variable in the Inspector
2) In the variable in the inspector, click the down arrow (where it says "[empty]" and choose "Load", then choose the scene you want