I can not change version

[José Luis]

could not calculate the update

a problem while the update is calculated has occurred.

That has been caused by :
* Upgrade to a version of Ubuntu yet published
* Be running the current version , not yet published , Ubuntu
* Unofficial software packages not provided by Ubuntu

If none of this applies , report this error using the command
' Ubuntu -bug ubuntu -release- upgrader -core ' in a terminal.

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: ubuntu-release-upgrader-core 1:0.220.8
ProcVersionSignature: Ubuntu 3.16.0-67.87~14.04.1-generic 3.16.7-ckt24
Uname: Linux 3.16.0-67-generic x86_64
ApportVersion: 2.14.1-0ubuntu3.19
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: GNOME
Date: Fri Apr 22 07:03:33 2016
InstallationDate: Installed on 2015-07-29 (268 days ago)
InstallationMedia: Ubuntu 14.04.2 LTS "Trusty Tahr" - Release amd64 (20150218.1)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
UpgradeStatus: Upgraded to trusty on 2016-04-22 (0 days ago)
mtime.conffile..etc.update.manager.release.upgrades: 2016-04-21T19:22:04.404490

[Brian Murray (brian-murray)]

Reviewing the files from your upgrade attempt you seem to have enabled a PPA that provides Xorg packages. This will prevent the upgrade as that PPA contains different versions of packages essential to Ubuntu. The page, https://launchpad.net/~xorg-edgers/+archive/ppa, contains details on to how to revert to the official Xorg packages for the xorg-edgers PPA. Instructions for reverting to official Xorg packages should be similar for the PPA that you have enabled.