I ran the command sudo apt-get install update and as you can see my results below:

cyberwizard34@Hogwarts:~$ sudo apt-get install update
[sudo] password for cyberwizard34: 
Reading package lists... Done
Building dependency tree       
Reading state information... Done
E: Unable to locate package update

Ubuntu 20.04 Focal Fossa uses - like its ancestors - apt

Before you can upgrade your running system you need to update the package-cache for the repositories that are enabled on your system, by using:

sudo apt update

Once that is done, you have to upgrade - meaning applying the newly found patches to your system, by using:

sudo apt full-upgrade

This should be it, except if you are using packages that are not supported anymore and/or you made manual changes to packages that were regularly installed via apt from your current repositories/sources; In that case, you would have to use:

sudo apt upgrade

The difference to sudo apt full-upgrade is that sudo apt upgrade does not remove packages from your system; which would be done for example because those non-standard packages reached end-of-life, were replaced by another (meta-) package or do break the dependency chain, which you might have willfully ignored.

If you are new to Ubuntu, Distros based on Debian (using apt as a package manager) or in general to GNU/Linux Distributions, you can ignore the alternative method, namely sudo apt upgrade, just use the first two commands mentioned:

sudo apt update && sudo apt full-upgrade

[ The && just chains commands, so you can write/copy-paste this one-liner.]

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