How to downgrade kernel after bad update (16.04)

Remove the latest kernel by

sudo apt remove linux-image-4.4.0-91-generic linux-headers-4.4.0-91-generic

That will uninstall meta packages linux-generic, linux-image-generic and linux-headers-generic. Kernels will never be upgraded.

To revert it you can run

sudo apt install linux-generic

That will install the latest kernel and kernels will get upgrades.

linux-generic is a meta package. It is empty, but depends on two other meta packages: linux-image-generic and linux-headers-generic.

The last two point to the latest kernel image and headers.

If you remove meta packages, nothing will be removed from the Linux, but kernel image and headers won't get updates. You can always install them later. They will pull the latest "real" kernel packages.

So the solution is to manually remove the kernel packages you don't want and also the meta packages.

Although Pilot6's solution may be the best solution, you can do less drastic measures.

I would personally remove the kernels you do not want from software center and then put those packages on hold - see How to prevent updating of a specific package?

This is a trivial change and you can over ride or revert the change rather easily.