update

To be clear, software update generally means to install updates to the macOS currently running on the Mac; Catalina 10.15.1 to Catalina 10.15.7, for example. User @Magmar has posted above the steps for updating your currently installed macOS 10.11 El Capitan.

A software upgrade is a generational leap forward to a newer macOS; from 10.11 El Capitan to 10.13 High Sierra for example, or 10.14 Mojave to 10.15 Catalina.

How to Upgrade  macOS:

The macOS version you can upgrade to depends on the model and year of your Mac.

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

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

Learn How to Back up your Mac with Time Machine:

https://support.apple.com/en-us/HT201250

❗️ – IMPORTANT – Your 32-bit apps won’t work in Catalina.

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

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

St. Claire Software – Go64: https://www.stclairsoft.com/Go64/

🖥 – Check 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 > High Sierra, Mojave, Catalina.

This guide explains how to update your Mac. I recommend to always update to the newest version. You will get the latest features and bug-fixes, and most importantly: the latest security updates.