I tried this:

$ sudo apt-get install python2.7.11
Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Unable to locate package python2.7.11
E: Couldn't find any package by regex 'python2.7.11'

But there is a python2.7.11 online. Why this error?

Then I do this:
$ sudo apt-get install python2.7.9

and the install seems to work.
Meaning no errors.

But when I checked, I have this python:

$ python
Python 2.7.6 (default, Jun 22 2015, 17:58:13)
[GCC 4.8.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.

Why am I not able to upgrade to python2.7.9 ?

I also tried to uninstall python2.7.6, but stopped when it shows me a long list of dependencies it will have to remove!

Is it a good idea to uninstall python?

Thanks.

apt-get install python=version
see man page

$ sudo apt-get install python=2.7.11
Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Version '2.7.11' for 'python' was not found
$ sudo apt-get install python=2.7.9
Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Version '2.7.9' for 'python' was not found

http://superuser.com/questions/39368...install-on-apt
probably you need an apt-get update