I am trying to install Skype for business and Microsoft office on my MacBook Air 2015. but need to update to macOS 10.11 for this.
I don't have any installation updates pending, how can I upgrade?
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Major releases need to be downloaded from the App Store. Just go there, it probably is shown on the main page (otherwise search for "Big Sur").
It is recommended to make a backup of your system first before starting the upgrade. Apple has more details in https://support.apple.com/en-us/HT201475.