You may wish to consult the official Python Software Foundation's guide.
Hopefully, your script was written with both Python 2.7 and 3.x in mind. The Python-Future's cheat sheet gives practical tips.
If your script does not contain
from future import division you will need to very carefully check all of the integer division operations in your code to ensure that results match between Python 2.7 and Python 3.x.
There are some known script changes that are needed if your script makes use of tkinter or winreg, as described below.
For Tkinter, please replace
import Tkinter with the following:
import tkinter as Tkinter
import winreg as _winreg