What do you mean by "will not upgrade"?
Is there an installation error?
In the help/about dialog does it say version "6.x.x" instead of "2015.x.x"?
Since LR6 and LR5 are side-by-side installs, are you starting the correct version?
In the Creative Cloud application, sign out. Then sign in using the userID/e-mail address that you used whne you subscribed to Lightroom CC