The Linux Kernel is the core of the Ubuntu operating system.The Linux Kernel is a monolithic Unix-like kernel for computer operating systems, it has been created by Linux Torvalds and is used by all Linux distributions including Ubuntu, Cent OS, Open Su SE, Read Hat and Debian.

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Here you can download complete kernels or kernel patches.

On writing this article the most up to date kernel is

Initially designed only for 386/486-based computers, now Linux supports a wide range of architectures, including 64-bit (IA64, AMD64), ARM, ARM64, DEC Alpha, MIPS, SUN Sparc, Power PC, as well as Amiga and Atari machines.

Looking at the appended shortlog, Linux kernel 4.5.2 adds minor improvements to the Btrfs, Overlay FS, NFS, and EXT4 filesystems, as well as to the ARM, ARM64 (AArch64), MIPS, PA-RISC, Power PC (PPC), s390, and x86 architectures.

Core kernel, crypto, and sound changes are also present.

Moreover, the networking stack received various enhancements as well, in particular for things like IPv6, IPv4, DSA (Distributed Switch Architecture), Datagram Congestion Control Protocol (DCCP), Layer 2 Tunneling Protocol (L2TP), the mac80211 framework, Multiprotocol Label Switching (MPLS), and the XFRM IP framework.

After removing old kernels, new boot menu will be available in next reboot.

In a healthy Debian system, each time you install a package or update your system, you will be prompted that old kernels (if any) can ber removed. The following package was automatically installed and is no longer required: linux-image-extra-4.4.0-42-generic Use 'sudo apt autoremove' to remove it.

However, disk space is not a problem in modern systems. It is recommended to keep at least one or two older kernels, so you can boot your system in an emergency situation (hardware or software compatibility issues with the current kernel).