I was foolish to upgrade my computer to lucid, and i got into a lot of errors, it will not boot proper the display.
This is why i want to downgrade back to Karmic.
Do you know how can you do this?
^agreed. However, I was able to re-install without losing essential data. I have explained it here: http://www.khattam.info/2010/03/15/howto-downgrading-from-ubuntu-10-04-lucid-lynx-to-9-10-karmic-koala/
Restore from the backup you made before upgrading? You made a backup right?
Seriously, you probably need to backup your data/configuration format and reinstall. There is no supported way on a Debian/Ubuntu based system to downgrade.