If you have not done any kernel upgrade before, then use the easier method below:
1. open synaptic
2. in search field, type in something like
3. then let it search, you should see a list of kerner (linux-image-xxx-yyy)
4. find those that are
choose one and install.
5. during installation look at what it says in synaptic..
6. when complete, reboot, and you should see the PAE kernel appear as the first line in grub start up menu.