How can I use a different version of Python for the Save & Run button?

The Save & Run button defaults to Python 3.6. If you want to use Python 2, you can do so by inserting a special line at the top of your file, called a hashbang:

For example:

#!/usr/bin/python2.7
print "hello from python 2"

Will load Python 2,

Or, for example:

#!/usr/bin/python3.4
print("hello from python 3.3")

will load python 3.4 instead of 3.6.

NB the hashbang has to be the very first line in the file.