Connecting nodes from the editor is now done in the Node dock. Select the node you want to connect a signal from, and then see what you can connect in the Node dock.
Changing the texture of a sprite can be done in several ways depending on your setup.
If you have two images in two files, you can set another texture in script this way:
var texture = preload("res://path/to/your/image.png")
I'm not sure how your scene is setup so I can't really tell where to put the code above in context. What I usually do is that I make my characters have a physics node as root with a script having a
touched_by_thing function. The sprite node is a child of it, and then I do this in the area2d:
(It's just one way of doing it, among others)