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To be clear, software update generally means to install software to update the macOS currently running on the Mac; update Catalina 10.15.1 to Catalina 10.15.7, for example.

To update your current macOS, click the Apple  icon > About This Mac and then click Software Updates.... If there are any updates available for your Mac, they will be listed and you can apply them when you like.

A software upgrade is usually a generational step up to a newer macOS; upgrade from 10.13 High Sierra to 10.14 Mojave, for example, or from 10.15 Catalina to macOS 11 Big Sur.

Do NOT upgrade until you have made a Time Machine or other backup of your startup drive.

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 left of your screen and choose About This Mac.

macOS 10.14 Mojave is compatible with the Macs listed here:

https://support.apple.com/kb/SP777?locale=en_US

macOS 10.15 Catalina is compatible with the Macs listed here:

https://support.apple.com/en-us/HT210222

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

https://support.apple.com/en-us/HT211238

❗️ – IMPORTANT – Backup your data, now, before you attempt an upgrade.

Learn How to Back up your Mac with Time Machine:

https://support.apple.com/en-us/HT201475

❗️ – IMPORTANT – NONE of your 32-bit apps will work in Catalina or Big Sur.

https://support.apple.com/en-us/HT208436

Before you upgrade, use this free utility to check the compatibility of your apps:

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

🖥 – Check the compatibility of your Mac with newer versions of macOS.

https://support.apple.com/en-us/HT201475

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

Click > Mojave, Catalina or Big Sur.

Once downloaded, double-click the installer app in the Applications folder to begin the upgrade.