TextureButtons emit the
pressed() signal when they are pressed. For a brief overview of signals, see the documentation.
You want to link this signal to some change scene code so that when the button is pressed, the change scene code is executed.
To do this, click on your
Button node, then select the Node tab beside the Inspector tab. Then select the
pressed() signal. This will open a window to select which node you want to connect the signal to. Select whatever main node is running the main script of your scene.
Then you can write whatever change scene code you want to be executed.