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/HT201475
macOS 11 Big Sur is compatible with the Macs listed here:
https://support.apple.com/en-us/HT211238
NOTE- If your Mac is not compatible with any of the upgrades listed above, then there is no other upgrade path but to buy a new or newer Mac that can run a newer macOS version.
❗️ – 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/HT201250
❗️ – 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
✅ - 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.
