Every time Ubuntu installs a new Linux kernel, the old one is left behind. This means that if you are regularly updating an Ubuntu system the Grub boot menu becomes longer and longer with kernels you don’t need anymore.
The old kernels are deliberately left installed and on the menu so you can boot a previous kernel if you have trouble with a new one. But if the new one works, you can safely uninstall the old kernel, which will also result in the Grub menu being cleaned up.
First you need to find out what your current kernel is. Open a terminal and run the following command: