I need to update MacOS for my MacBook (Retina, 12-inch, 2017) to install new apps. When I try to download, message said that there is no update available. Please someone help me of how to download and install macOS.

That Mac can upgrade all the way to Ventura. The App Store likes to hide old versions of macOS by default, so you will probably need to use one of the App Store links in this article: How to download and install macOS - Apple Support

If you upgrade to Catalina or later,

• 32-bit applications will stop working
• iTunes will go away, replaced by separate applications: What happened to iTunes? - Apple Support

Here is information which addresses your confusion. It is important to understand the difference between updates and upgrades. An update is when we install a newer version of the same operating system. It will be found in the System Preferences by clicking the Software Update button. If you already have the latest version installed, you will be notified that no update is available which is apparently your case. An upgrade is when you install a newer operating system such as upgrading from Catalina to Big Sur. What you can install is dependent on the year and model Mac you have. Upgrade links may be found in the Apple App Store. I hope this information is helpful in clearing up any confusion.

You are probably at the end of the update cycle for your Mac OS version.

The maximum you can upgrade to is Ventura. Keep in mind if you are running Mojave or less (Sierra is less), your Mac is at the max OS that supports 32 bit applications. You will probably need to upgrade a bunch of Apps if you want to install Catalina, Big Sur, Monterey or Ventura.

Ventura's October 24, 2022 release! - Apple Community