unable to update ubuntu 14.04 to 15.04

[Kanchan]

unable to update ubuntu 14.04 to 15.04

getting following error message

An unresolvable problem occurred while calculating the upgrade.

This can be caused by:
* Upgrading to a pre-release version of Ubuntu
* Running the current pre-release version of Ubuntu
* Unofficial software packages not provided by Ubuntu

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: ubuntu-release-upgrader-core 1:0.220.8
ProcVersionSignature: Ubuntu 3.13.0-75.119-generic 3.13.11-ckt32
Uname: Linux 3.13.0-75-generic i686
NonfreeKernelModules: wl
ApportVersion: 2.14.1-0ubuntu3.19
Architecture: i386
CrashDB: ubuntu
CurrentDesktop: Unity
Date: Sat Jan 16 12:12:43 2016
InstallationDate: Installed on 2015-01-04 (376 days ago)
InstallationMedia: Ubuntu 13.10 "Saucy Salamander" - Release i386 (20131016.1)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to trusty on 2016-01-16 (0 days ago)
mtime.conffile..etc.update.manager.release.upgrades: 2015-09-08T16:24:32.521385