We'd like to get your operating system updated so you can receive the latest security patches.
Look for the Bionic Cat icon in your menubar toward the top-right of your screen (near the time) and select "install update."
Select install at the left and search for or find "Install macOS Big Sur" and click "install." (Note that the screenshot is from a machine that already has Big Sur, but your button will read "install" and be clickable.)
After clicking "install," your computer will download the update in the background and notify you when it's time to install.
Be forewarned that, once you click install, it can take 45-60 minutes to install.
Also, please sure to keep your computer plugged into power and the laptop lid open while the upgrade runs.