Facebook says my safari browser needs to be updated on my MACBOOK. how do I update it?

As MrHoffman wrote.

Update to the latest version of Safari - Apple Support

Excerpt:

"If a website says that Safari is out of date even though you're already using the latest version of macOS, iOS, iPadOS, or visionOS, there could be an issue with the website. If you’re sure that you want to use the website, contact the website owner or developer for guidance about how to best use their website."

Your browser is fine, Facebook is malfunctioning. Check with Meta Support for assistance.