I have an iMac (Retina 5K, 27-inch, 2017) using macOS High Sierra and I want to know how to upgrade to newer system.

❗️ Do NOT upgrade…

until you have made a Time Machine or other backup of your startup drive.

Learn How to Back up your Mac with Time Machine

How to Upgrade  macOS:

The macOS version you can upgrade to depends on the year your Mac was released.

Click the  (Apple) menu at the top of your screen and choose About This Mac.

Check the Compatibility of your Mac with newer versions of macOS:

➡️ macOS 10.14 Mojave is compatible with the Macs listed here

➡️ macOS 10.15 Catalina is compatible with the Macs listed here

➡️ macOS 11 Big Sur is compatible with the Macs listed here

➡️ macOS 12 Monterey is compatible with the Macs listed here

➡️ macOS 13 Ventura is compatible with the Macs listed here

❗️ – IMPORTANT – None of your 32-bit apps will work in macOS Catalina or newer. This includes MS Office apps and any other older software apps and drivers. These will require that you find 64-bit updates and/or replacement titles.

32-bit app compatibility with macOS High Sierra 10.13.4 and later - Apple Support

Use the free utility Go64 to check the compatibility of your apps:

https://www.stclairsoft.com/Go64/

✅ - IF your Mac is compatible, then download the macOS upgrade installer app:

Click > Mojave,  Catalina,  Big Sur,  Monterey  or  Ventura.

Once downloaded, double-click the “Install macOS…” app in the Applications folder to begin the upgrade.

Please see > How to download macOS.