How do I upgrade my MacBook Air from High Sierra 10.13.6 to Big Sur 11.0?

If your specific model of MacBook Air is compatible with macOS 11 Big Sur, you can download the installer from the following Apple support document.

click here ➜ How to download and install macOS - Apple Support

click here for compatibility ➜ macOS Big Sur is compatible with these computers - Apple Support

macOS Big Sur - Technical Specifications

General Requirements

OS X 10.9 or later

4GB of memory

35.5GB available storage on macOS Sierra or later*

Some features require an Apple ID; terms apply.

Some features require a compatible internet service provider; fees may apply.

Mac Hardware Requirements

For details about your Mac model, click the Apple icon at the top left of your screen and choose About This Mac. These Mac models are compatible with macOS Big Sur:

MacBook (2015 or later)

MacBook Air (2013 or later)

MacBook Pro (Late 2013 or later)

Mac mini (2014 or later)

iMac (2014 or later)

iMac Pro (2017 or later)

Mac Pro (2013 or later)

Learn about compatibility and how to identify your specific Mac model

https://apps.apple.com/us/app/macos-big-sur/id1526878132?ls=1&mt=12

And caution on 10.15 or higher, to see which Apps will no longer work in 10.15 get Go64...

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

Click on the Apple menu-->About This Mac. This will tell you which Mac model you have (include the year info, like Late 2015 or something like that).

Next, look at this MacWorld article that details what MacOS your computer is capable of: MacWorld Article

Note that if you can upgrade your Mac to Catalina or later – and do – 32-bit applications will stop working.