How to update kernel to specific version

Warning: doing things below is dangerous if you care about system security. Proceed only if you are sure what do you want to achieve.

So you have to remove meta-package (it usually depends on the latest kernel, nowadays it is 5.4.0-86 in this HWE stack) by

sudo apt-get update
sudo apt-get remove linux-image-generic-hwe-18.04
sudo apt-get autoremove

and then install fixed version by

sudo apt-get install linux-image-5.4.0-81-generic

But please again note that latest kernel is safer, it may have many CVEs fixed. Check changelog for details.

So to get all latest upgrades back you have to execute the following commands:

sudo apt-get update
sudo apt-get install --install-recommends linux-generic-hwe-18.04  
sudo apt-get dist-upgrade # to get all latest dependencies

and then reboot to use all latest software including the kernel.

More stuff to read:

• man apt locally or online, man apt-get locally or online
• https://wiki.ubuntu.com/Kernel/LTSEnablementStack#Ubuntu_18.04_LTS_-_Bionic_Beaver
• https://help.ubuntu.com/18.04/serverguide/package-management.html