Trying to update Kubuntu from 12.04 LTS to 14.04 LTS with kubuntu-devel-release-upgrade

[Exagone313]

The paquet update-manager is not curently installed (?).

Solution in my comment.

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: update-manager (not installed)
ProcVersionSignature: Ubuntu 3.2.0-67.101-generic 3.2.60
Uname: Linux 3.2.0-67-generic x86_64
NonfreeKernelModules: nvidia
ApportVersion: 2.0.1-0ubuntu17.6
Architecture: amd64
Date: Thu Aug 7 13:31:07 2014
InstallationMedia: Kubuntu 12.04 LTS "Precise Pangolin" - Release amd64 (20120423)
MarkForUpload: True
ProcEnviron:
 LANGUAGE=fr_FR
 TERM=xterm
 PATH=(custom, no user)
 LANG=fr_FR.UTF-8
 SHELL=/bin/bash
SourcePackage: update-manager
Symptom: ubuntu-release-upgrader-core
UpgradeStatus: Upgraded to precise on 2014-08-07 (0 days ago)
VarLogDistupgradeTermlog:

[Exagone313 (exa)]

It solved the problem :

Edit all configurations of sources : replace "precise" by "trusty" (useless for cd sources).
Move all files in /etc/apt/sources.list.d/ in another folder (example a "bak" folder within it will be ignored).
Execute the following commands :
sudo apt-get update
sudo apt-get upgrade
sudo reboot

After the reboot, use that command to check if you have Ubuntu 14.04 installed :
lsb_release -a

My output :
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 14.04.1 LTS
Release: 14.04
Codename: trusty