This article shows how you can update SharePoint 2019 using Cumulative Updates. The information in this document don’t apply to SharePoint 2013 or earlier versions.
Table of contents:
How to: SharePoint 2019 Updates E-Book:
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This E-Book explains the update process for SharePoint 2019 in a simplified but comprehensive way.
There is lots of great information out there but often located at many different places: TechNet, Videos, Blogs or even comments in blog posts. It takes time to find them and bring them in the right relation to each other.
You can easily miss important steps in your update process. My goal is to give you an easy way to get started.
All information is linked with resources if you need additional, more detailed explanations.
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There are 3 important types of updates :
The naming varies across different resources and the term "Cumulative Updates", which is known from SharePoint 2013, has been replaced by "Public Updates" if you read Microsoft’s documentation.
Stefan Goßner still uses the term Cumulative Updates and if you read his blog it becomes clear that there is a difference between the updates which are released.
Service Packs include older Service Packs, Cumulative Updates, Public Updates and new fixes or features.
Microsoft didn’t release Service Packs for SharePoint 2016 to "ensure that customers are running a build that contains fixes for the most common issues".
Although Microsoft didn’t update their documentation for SharePoint 2019 it’s likely that this policy won’t change.
Microsoft has no plans to release Service Packs for SharePoint 2019 so you should install Cumulative Updates and Public Updates instead. 
It’s a rollup update which means that the newest CU includes new fixes and all previous fixes (CUs as well as Public Updates) that were released after a major version e.g. RTM or SP1.
Microsoft uses the name Public Update for this rollup update but the name Cumulative Update makes more sense.
Cumulative Updates are released every month.
Yes! Usually you install them:
Usually you need to have the latest Service Pack installed (if available) to install the newest CU.
You always have to download 2 (!) files:
But here is the trick: This file is multilingual which means that it doesn’t matter which language you select on the download page. Your download will include fixes for all(!) SharePoint languages. Just select a language on the download page you can read.
Some of the Cumulative Updates mentioned in the previous section include security fixes. If a CU contains a security fix the language independent file (which fixes core components) is made available through Windows Update so it can reach a much wider audience:
A Public Update is the exact same language independent file which is part of a CU. The only difference is that it is made available through Windows Update because it contains a security fix.
So, does it make sense to talk about CUs and PUs?
Because it is the same language independent file... and because TechNet only uses the term Public Update for all updates. It is up to you but in my opinion it makes sense:
All information from the previous section about Cumulative Updates applies..
All information from the previous section about Cumulative Updates applies.
Via Windows Update.
Note: You don’t have to download the language dependent file if you install a PU through Windows Update. Different components of SharePoint server can have different patch levels.
Let me give you a few examples when to install which update:
A Feature Pack is just a "fancy name" for a cumulative update .
If you need product support from Microsoft you must have a specific patch level before Microsoft starts helping:
"All SharePoint Server 2019 builds will be supported for at least one year from its release date. Microsoft will update the minimum supported build of SharePoint Server 2019 on each anniversary of General Availability (GA). Customers must be running that build or higher to remain supported. If a customer contacts Microsoft support and their farm is not running the minimum supported build or higher, they will be asked to upgrade to that build before support can be offered." 
I wrote in a previous section that "different components of SharePoint server can have different patch levels". But how do you reliably get the patch level of your SharePoint farm?
Stefan Goßner has a very detailed article  about all the different version numbers you can find in SharePoint.
There is only one reliable way to know the correct patch level of your SharePoint environment:
Go to: Central Administration > Upgrade and Migration > Check product and patch installation status
Patch Level Example
If you install February 2019 Public Update through Windows Update on SharePoint 2019 RTM, you’ll have the following patch level:
As you can see in the picture above the February 2019 Public Update offered through Windows Update is exactly the language independent file of the February 2019 Cumulative Update. The language dependent file of the February 2019 Cumulative Update is not installed via Windows Update.
If you want to install the complete February 2019 Cumulative Update, you can download the language dependent file and install it on your own:
If you didn't successfully install SharePoint 2019 please do this first.
Please make sure that you meet SharePoint 2019 hardware and software requirements  and that you run SharePoint servers with enough available hard disk space and RAM.
Before you update SharePoint, you need to make sure you use an account that meets the following requirements :
Usually the SharePoint Setup Administrator meets these requirements. It’s the account you used to install and configure SharePoint in the first place.
Make sure that:
If you want zero downtime during your update you need to make sure that:
You don’t need to use the MinRole feature but you need to make sure your servers and all your services are highly available.
Here is an example  if you use the MinRole feature: