This is the most easiest way to upgrade Ubuntu from any version to any latest Ubuntu Version. You can upgrade with just few clicks and rest of the process Ubuntu will do.
Open "Dash" and Search for "Update Manager" then open it. In update manager Click on "Settings" Button.
Also Disable all Third-Party repositories from other software Tab.
A new window will open, Go to "Updates" Tab at the end of this window "Notify me of a new Ubuntu Version" Select "For any new Version" and click ok, then close the update manager.
Now start "Update Manager" again and Click on "Upgrade" Button. That's it
Here is command line upgrade