In order to change to default python version on your Raspbian GNU/Linux first list all available python versions:
# ls /usr/bin/python*
/usr/bin/python  /usr/bin/python2  /usr/bin/python2.7  /usr/bin/python3  /usr/bin/python3.2  /usr/bin/python3.2mu  /usr/bin/python3mu
Your output may be different. If the version you are looking for is not available then use apt-get command to install it. eg. apt-get install python3 Next setup alternatives:
# update-alternatives --install /usr/bin/python python /usr/bin/python2.7 1
update-alternatives: using /usr/bin/python2.7 to provide /usr/bin/python (python) in auto mode
# update-alternatives --install /usr/bin/python python /usr/bin/python3.2 2
update-alternatives: using /usr/bin/python3.2 to provide /usr/bin/python (python) in auto mode
Since we have setup python3.2 with a higher priority it should be now your default:
# python --version
Python 3.2.3
To swap between python version use update-alternatives command:
# update-alternatives --config python
There are 2 choices for the alternative python (providing /usr/bin/python).

  Selection    Path                Priority   Status
------------------------------------------------------------
* 0            /usr/bin/python3.2   2         auto mode
  1            /usr/bin/python2.7   1         manual mode
  2            /usr/bin/python3.2   2         manual mode

Press enter to keep the current choice[*], or type selection number: 1
update-alternatives: using /usr/bin/python2.7 to provide /usr/bin/python (python) in manual mode
Your python version should now be changed to reflect the above selection:
# python --version
Python 2.7.3
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