When attempting to upgrade from ubuntu 12.04 to 14.04, it crashes? Why is that and how can that be resolved?

[Bruce Hoo Fung]

When doing the upgrade, it states an unresolvable problem occurred while calculating the upgrade. This is 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 if none of this applies, then please report this bug using the command 'ubuntu-bug ubuntu-release-upgrader-core' in a terminal

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: update-manager 1:0.156.14.21
ProcVersionSignature: Ubuntu 3.2.0-117.159-generic-pae 3.2.79
Uname: Linux 3.2.0-117-generic-pae i686
ApportVersion: 2.0.1-0ubuntu17.13
Architecture: i386
Date: Mon Dec 5 09:34:40 2016
GsettingsChanges:
 com.ubuntu.update-manager first-run false
 com.ubuntu.update-manager launch-time 1480947912
 com.ubuntu.update-manager window-height 500
 com.ubuntu.update-manager window-width 600
InstallationMedia: Ubuntu 12.04.1 LTS "Precise Pangolin" - Release i386 (20120817.3)
MarkForUpload: True
PackageArchitecture: all
ProcEnviron:
 LANGUAGE=en_US:en
 TERM=xterm
 PATH=(custom, no user)
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: update-manager
Symptom: ubuntu-release-upgrader-core
UpgradeStatus: Upgraded to precise on 2016-12-05 (0 days ago)
VarLogDistupgradeTermlog: