First things first: Back up everything you can't afford to lose.
Disable any PPAs and non-standard sources you may have in your sources list. Uninstall any proprietary drivers.
Use the terminal to do
Code:
sudo apt-get update
and
Code:
sudo apt-get dist-upgrade
dist-upgrade does not do the upgrade to the new version. It does what upgrade would do but it also attempts to resolve dependency issues.
Then see if you have the option to upgrade to 12.04.