How do you download the Safari Update for Mac?

What system are you running now?

Recent MacOS versions manage OS upgrades and updates from the App Store - just launch the App Store on your system and check the Updates section.

The App Store will automatically give you the latest version available for your system. If your system is old then you may not be able to run the latest version.

Then upgrade your OS.

Safari is tied to, and bundled with, OS updates. As you update the OS running your system you automatically get the latest compatible Safari version.