The kernel of Ubuntu is in the package
linux-image-$version, to see what packages of this type you have installed, do in a terminal the following command:
dpkg -l | grep linux-image
The update of kernel package, like of any other packages, will be automatically proposed by the system, if available.
If you suspect the update manager is not working properly, execute the following command:
sudo apt-get update
sudo apt-get dist-upgrade
and post any errors they show.
If you otherwise need to upgrade to a newer kernel not available in lucid repos, you can download and install that of your choice on
but be aware that they are not specifically tested against your system.