To be clear, in the world of Macs, software update is a revision of 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. Software updates may be set to occur automatically. If no updates are presented, then it may be assumed that none are available for your current OS.
A software upgrade replaces the current macOS with a newer, differently named version; upgrade from 10.13 High Sierra to 10.14 Mojave, for example, or from 10.15 Catalina to macOS 11 Big Sur. Upgrades do not happen automatically, They must be initiated by a user with admin privileges.
Use Software Update to update or upgrade macOS, including built-in apps like Safari.
Please see >: Update macOS on Mac - Apple Support
❗️ 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
❗️ – 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 64-bit compatibility of your apps:
https://www.stclairsoft.com/Go64/
✅ - IF your Mac is compatible, then download the macOS upgrade installer app:
Click > Mojave, Catalina.
Once downloaded, double-click the “Install macOS…” app in the Applications folder to begin the upgrade.
Please see > How to download macOS.