Kernel question on when to update

[Pliny.D.Elder]

dpkg -l | grep linux-image

That will give you kernels installed.

Mine looks like this....

i linux-image-4.4.0-51-generic 4.4.0-51.72 amd64 Linux kernel image for version 4.4.0 on 64 bit x86 SMP
ii linux-image-4.4.0-53-generic 4.4.0-53.74 amd64 Linux kernel image for version 4.4.0 on 64 bit x86 SMP
ii linux-image-4.4.0-57-generic 4.4.0-57.78 amd64 Linux kernel image for version 4.4.0 on 64 bit x86 SMP
ii linux-image-extra-4.4.0-51-generic 4.4.0-51.72 amd64 Linux kernel extra modules for version 4.4.0 on 64 bit x86 SMP
ii linux-image-extra-4.4.0-53-generic 4.4.0-53.74 amd64 Linux kernel extra modules for version 4.4.0 on 64 bit x86 SMP
ii linux-image-extra-4.4.0-57-generic 4.4.0-57.78 amd64 Linux kernel extra modules for version 4.4.0 on 64 bit x86 SMP
ii linux-image-generic 4.4.0.57.60 amd64 Generic Linux kernel image

As you can see I have 3 installed. I like to keep 2 so I would in a terminal

sudo apt --purge remove linux-image-4.4.0-51-generic linux-image-extra-4.4.0-51-generic

Hope that helps.