make a variable that is a number like:
var counter = 0
then you can change it freely, like:
counter += 10
and if you want to update the text label use:
YourLabel.text = str(counter)
YourLabel is of course your node and str function converts a number to string, which can be used with text fields.
Also, if you don't know how to get YourLabel handle, you can do this by two ways:
in both cases Godot should give you tips as you start writing.