How do I learn Python 2 if I already know Python 3?
If you already are familiar with Python 3, then there are almost no differences you will have to worry about when coding in Python 2. The most user-visible differences have to do with details of the print statement, which you probably won't be using for Django anyway.
So, just write code, and ask about any specific problems you might encounter.
I have some knowledge of Python 3 (I'm not a beginner, but I'm not an expert). I'm interested in web development, so I want to use Django. What are the differences between the two versions of Python? How should I switch from 3 to 2.x?