Since it have a good rank on google i'll answer it, the reference documentation doesn't have an exemple and it require to understand signaling.
var timer = null
# Called when the node enters the scene tree for the first time.
timer = Timer.new()
timer.connect("timeout", self, "on_timeout")
in timer.connect() the first argument is the signal (the only signal Timer is emitting is "timeout". Self it the receiver of the signal, and ontimeout is a function name of your choice, i called it "ontimeout" but could have been anything.
And in this function call the change_scene code.