Please specify exactly what you need because now i feel like you ask for one thing in title of that post and for another thing in content of post.
But I will try to answer both of them:
Is it possible to upgrade AE CS6 to AE CC 2015?
Yes. First you have pay for Adobe CC subscription to have access to newest versions of AE - Mylenium gave you link to that already. Then you have uninstal CS6 and instal CC - thats it.
I am trying to get this file to work on AE CS6 but I need AE CC for it to work.
If I understand you correctly you have file prepared in newer version of AE (CC) but you have CS6 version - is that correct? If that is the case and as AE have no backward compatibility (the only way is to save file from newer version to older - but only one step/version back) you have two ways to work with that file:
a) as I and Mylenium says: just buy CC subcription and use nevest version of AE
b) ask someone who have never version of AE to save that file for you with compatibility with CS6 version of AE
A lot of people including me are incredibly unhappy with CC and are downgrading to CS6.
Honestly if you don't need one of the few new features added in CC, don't bother upgrading. CC 2015 is full of bugs and doesn't work properly.
omid_pakbin - yes, theres some people that don't need to upgrade to CC, there's also some people have some issues with newer versions - most don't but if you work with projects from someone else (as the case is in that topic) you have only two chices: upgrade to CC or have someone who will downgrade projects made in never versions for you. So, no one forces anyone to anything but please read what is the real issue in futons question and try to help him before you start another discussion on how some like or dislike CC and love or hate CS6 - we have plenty of those.