I need to update my MacBook but I do not have the software update link.

The build model year of your Mac is key to whether or not it could use a newer macOS.

And then, that could allow a path to different system version; an upgrade past the one

your Mac came with and perhaps then also have more update steps, for further use.

There are additional systems, from 'older to newer' among pre- Big Sur macOS. As

these are progressive; for awhile where they once held sway, some could be useful.

• How to get old versions of mac OS - Apple Support

https://support.apple.com/en-us/HT211683

If your MacBook identifies as '2015-2019 12-inch Retina' there may be some options.

Also a link to read about Big Sur is inside the above page; to access more info there.