How to update Xcode?

I have a Mac running High Sierra, version 10.13.6. When running MacPorts I get the following error message that I should update my Xcode installation:

Error: The installed version of Xcode (8.3.1) is too old to use on the installed OS version. Version 9.3 or later is recommended on Mac OS X 10.13.

Running xcodebuild -version confirms this:

Xcode 8.3.1

Build version 8E1000a

But when I launch App Store I can't find any updates.

Has anyone an idea how I can update Xcode including command line utilities?

You probably just install the current version from the MAS.

https://itunes.apple.com/app/xcode/id497799835?l=en&mt=12

The command line utilities are included within the Xcode bundle. On first launch symlinks to the utilities are installed into the system so they can be accessed from the Terminal/command line.

There is also a separate Command Line Tools package available from the Developer Account download archive (needs an Apple Developer Account).

https://developer.apple.com/download/more/?=for%20Xcode