Stubbbsy345's is correct, but I usually prefer to use getters and setters. When it is a simple variable, maybe it looks a bit stupid, but sometimes you want a more complex behaviour when you set or get a variable.
Also, it allows the use of duck typing, which is one of the best features of GDScript.
In the variable you want to instance:
var my_var = 5
my_var = value
instanced_variable.setVar(6) #Then, inside instanced_variable, my_var = 6
var my_var_from_instace = instanced_variable.getVar()