The latest version of macOS Server (5.4) is now available to be installed. To do so, first backup your server. Then, backup your server again, making sure you have a functional, bootable clone. Once you’re sure you have a solid backup of your server, open the App Store and search for Server. When you find the Server app, click on it. 
Once downloaded, you’ll be prompted that the Server app has been replaced.
Go into Applications and open the Server app. When prompted, click on Install (or Open if the server is already installed). 
The download will begin. Once complete, you’ll see a notice that the “Server app replacement detected.” Click OK. Then, open the Server app. When the Server app opens, you’ll be prompted to update the server. Click Continue. 
At the Licensing Agreement screen, click Agree. At the screen to confirm your administrative access, provide a name and password for an account with administrative access and then click on Allow. Services are then upgraded. Once complete, the Server app will open and should have settings consistent with the settings prior to the upgrade. 
