How do I revert back to a previous update of ubuntu

We don't what release of Ubuntu you're using (nor which product; Ubuntu Core? Ubuntu Server? Ubuntu Desktop?) nor what type of backup strategy you've put in place. If you're using a LTS release of Ubuntu (*we have no idea unless you tell us*), which kernel stack you've opted to use (*switching stacks can be an easy fix - but you've not given us any details of your product, release so we have no idea if this is an option for you*)

Just to make it clear for you. You are not selecting a previous version of the OS when you do that you are selecting a different kernel of the current release.

Can you access a terminal login screen using Ctrl+Alt+F3 after GRUB? What is your Ubuntu version? Do you have Nvidia graphics card?

I am running Ubuntu 20.04. It is the current kernel that is defective. I don't know the exact number of the kernel update, but it displays when grub comes up. When grub comes up, I have to catch it and go to Advanced Options and then scroll down past the recent kernel updates and select the last one that worked. I just wanted find a way to remove those recent kernel updates from the list so that the last update that worked will be at the top of the list and will boot automatically. If that is possible, then I would like to turn off updates to avoid future faulty updates.

And yes, my computer does use an Nvidia graphics card.

I can access Terminal after I have booted from the previous Kernel that worked, but the current Kernel only brings up a blank black screen.

Under Advanced Options on the grub screen, there are three kernel updates. The first two (5.11.0-37 and 5.11.0-36) are faulty and just bring up a black screen with an intermittently flashing cursor. The third one (5.8.0-59) is the one that actually works and allows me to boot Ubuntu. Is it possible to delete the two that begin with 5.11 so that I can automatically boot with 5.8.0-59, or is there a way to fix the most recent 5.11 so that it will actually boot?

5.8 is no longer a supported & patched kernel, so if your machine is on-line, you'd be better switching to the GA kernel which is patched (ie. using the supported 5.4 GA kernel over current 5.11 HWE kernel) assuming you're talking about Ubuntu 20.04 LTS (*your question doesn't say; additional details should be added to your question; comments are from us to you; not your replies if they contain additional detail - that belongs in your question*)

guiverc I am not quite sure what you are asking. All I know is that the 5.8 kernel is the last kernel that worked. That is what I am using right now. I have to manually select the 5.8 kernel, because if I allow my system to automatically boot to 5.11, it just goes to a blank dark screen with a random flashing cursor. I did specify that I am using Ubuntu 20.04 in the earlier post.

20.04 has been using the 5.11 kernel for some time, eg. https://fridge.ubuntu.com/2021/08/27/ubuntu-20-04-3-lts-released/ shows the ISO release date, but installed systems upgraded to it in the weeks before that ISO release; the 5.8 kernel was from 20.04.2 (being the 20.10 kernel; but 20.10 is now EOL; 5.11 is the 21.04 kernel stack). Ubuntu LTS have two kernel stack choices; GA = 5.4 for the life of 20.04, and HWE uses kernels from subsequent non-LTS releases; see https://wiki.ubuntu.com/Kernel/LTSEnablementStack , 5.8 kernel is *unsupported* & unsafe if you're online

Okay, but regardless of whether 5.8 is unsupported, I am forced to use it because 5.11 doesn't work. Is there any fix for 5.11? I can't boot into 5.11, because there is no display.

As I've already said; the GA or 5.4 kernel is patched & supported the life of the product, so if you can't use the HWE or 5.11 kernel - use the GA kernel following instructions int eh LTSEnablementStack link I gave last comment. 5.8 is fine if you're off-line, but not if you box is on-line. The GA is the *more* stable kernel for 20.04 which is why it's default for server installs (some desktop installs too depending on how installed); but the 5.8->5.11 kernel change was mid-late-August & not *most-recent*

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