Kernels do not update. You install a new kernel and both are present, you only uninstall the old one when you are sure the new one is working. With kernel updates it's a case of: "If it ain't broke, don't fix it" but in your case something is broke so it's worth trying.
In synaptic, search on Name only, for "kernel" and you can install the most recent one, either bfs, pae or a64 depending on what suits your hardware best. The default on reboot will be the old one, and if dkms modules are being built, it may take a long time on the first reboot (but after that return to normal).
Now you can try if the new kernel makes a difference. Then stick to it, unless there is a regression elsewhere.
You can uninstall the old kernel once you are happy with the new one.