http://www.linuxquestions.org/questions/debian-26/change-default-python-version-605397/
Well, if the link exists, remove it: "rm /usr/bin/python". Then make the link with ln -s
In my example output the files with a "->" pointing to a real file, are links. The links you can remove to change them.
You have python pointing to python2.4 . If you want 2.5 do:
rm /usr/bin/python
ln -s /usr/bin/python2.5 /usr/bin/python