How can I upgrade my iMac to the latest macOS?

 How to Upgrade to macOS 12 Monterey or macOS 13 Ventura

❗️– WARNING – Backup your data before proceeding. (You’ve been warned.)

Learn How to Back up your Mac with Time Machine:

Back up your Mac with Time Machine - Apple Support

❗️ – IMPORTANT – If upgrading from 10.14 Mojave or earlier, your older 32-bit 

apps & drivers won’t work in Monterey! They must also be updated or replaced.

Use this free utility> Go64 app from St. Claire Software to check your apps

🖥 – Check compatibility of your Mac:

macOS Monterey is compatible with these computers

macOS 13 Ventura is compatible with these computers

For details about your Mac model, click the Apple menu () at the top of your screen and choose About This Mac. 

Then do one of the following:

1) ✅ - Click on Apple menu  > System Settings… > Software Update.

If your Mac does not offer an upgrade in Apple menu  > System Settings…,download Monterey from the App Store. The download will place the "Install macOS Monterey" app in the Applications folder. Double-click the installer to begin the upgrade process. 

OR alternatively for Ventura:

2) ✅ - Click on Apple menu  > System Settings… > Software Update.

If your Mac does not offer an upgrade in Apple menu  > System Settings…, download Ventura from the App Store. The download will place the "Install macOS Ventura" app in the Applications folder. Double-click the installer to begin the upgrade process. 

Your tag line says the you have a 21.5" Retina 4K iMac. There were three sets of those.

• Monterey is the end of the line for the Late 2015 ones.
• Ventura is the end of the line for the 2017 ones.
• The 2019 ones can run Sequoia. However, if your tag line is correct and you are currently running some version of macOS 10.12 (Sierra), you do not have a 2019 model. Those originally shipped with macOS 10.14.4 (Mojave), and are unable to run any version of Sierra.

How to download and install macOS - Apple Support

If you upgrade your Mac to Catalina or later,

• 32-bit applications will stop working
• iTunes will go away, replaced by (Music, TV, Podcast, Books) plus Finder-based iPhone/iPad/iPod management

To identify your year model and latest macOS version it can run, go to > Identify your iMac model - Apple Support

macOS Sequoia requires a 2019 or later iMac.