It is really easy:

$ 2to3 -w example.py

where example.py is the file you intend to convert and -w means overwrite the original file.(But a example.py.bak is generated for reverence)

$ 2to3 example.py

Simply shows the difference in code between Python2 and Python3.

References

1. http://python3porting.com/2to3.html
2. https://docs.python.org/3.1/library/2to3.html