get_node() are interchangeable, but if you want to go up a level, it would be
$"..". To go up multiple levels, you'd use
get_tree().root will get you the root viewport, so you can get your main node from that:
get_tree().root.get_node("Main") for example.
All this said, it's typically a bad idea to be going up the tree. It means you're not going to be able to move nodes around because they expect a fixed parent structure. It's much more flexible to call down the tree, and use signals if you need to go up.