How to delete the old kernel after upgrading?

You better don't manually delete anything, remove it using your preferred tool.
For instance using apt-get: sudo apt-get remove name_of_old_kernel
One way to delete cached updates is to do a "sudo apt-get clean" from terminal.

You can also do it from synaptic package manager.
You can find your latest and old kernels from the section "base System" and mark them for removal.
But *dont* do it manually.

Hi! Just a tip:

By experience I tell you that you should keep the old kernel, just in case anything goes wrong, unless of course, you absolutely need the extra space on your disk. If something fails or is changed on the kernel configuration and you can't boot onto it, it's REALLY easy to reinstall it by booting from the old kernel (you can just use synaptic to reinstall anything with the name of your new kernel), but not so easy if you only had that kernel that was damaged.

Anyway, if you do remove it, as everyone said, DON'T do it manually! :D

You can remove old kernel version using System→Administration→Synaptic package manager and searching "linux-image".

I think it is always better to have 2 kernel so if one gives you problems you can always return the previous.

Hope this help.

until now there is no simple fix for this. there should be a tool for that

Please copy and paste hre the result of this terminal command:

dpkg -l | grep -i ^^ii | grep -i linux-image