You have a function that returns something so you need to assign it to a variable when you call it. This should do it:
#defining the variable
msg = "Hello"
#function to change the msg variable to the argument in the funtion
def changeit(new):
msg = new
return msg
#calling the function with the argument "goodbye"
msg = changeit("Goodbye")
#Printing the variable to check that it has changed acording to the function that we called
print msg