How to update a Mac?

That depends on what the current version is. On macOS versions for the last few years, software updates have been located in System Settings > General > Software Update. If it's older, macOS point updates (i.e. x.0.1 or x.1) are done via System Preferences > Software Update, and major upgrades (i.e. OS X Yosemite to El Capitan) are done through the App Store.

See Update macOS on Mac - Apple Support for more information.

"How to update a Mac?: does anyone know how to update a mac"

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About Updating a Mac:

Looking to upgrade to macOS Tahoe?

Determining which Update(s) will Install:

I. Verify Compatibility:

A. Start Here: Learn About Compatibility and how to Identify your Specific Mac Model - Apple Support

B. How to Verify:

II. Reveal your Model:

What is the Model of your Mac? To find out...

1. Go to: Apple menu
2. Click: About this Mac
3. View: Second Line-Down

then...

II. Compare the Results:

• macOS Tahoe is Compatible with these Computers - Apple Support
• macOS Sequoia is Compatible with these Computers - Apple Support
• macOS Sonoma is Compatible with these Computers - Apple Support
• macOS Ventura is Compatible with these Computers - Apple Support
• macOS Monterey is Compatible with these Computers - Apple Support
• macOS Big Sur 11 Hardware Requirements - Apple Support
• macOS Catalina 10.15 - Hardware Requirements - Apple Support
• macOS Mojave 10.14 - Hardware Requirements - Apple Support
• macOS High Sierra 10.13 - Hardware Requirements - Apple Support
• macOS Sierra 10.12 - Hardware Requirements - Apple Support

III. If Compatible:

• Back Up your Mac First:  Be certain you Create a Time Machine Backup of your Mac, so that way you can have something to restore your Mac from, should anything go wrong with the reconfiguration.

Learn how to ⇢ Update macOS on Mac: ⇣

Need more help?

• Call 1-800-APLCARE.
• Contact Support.
• Mac Support.
• Mac User Guide.

Good luck! 👋🏼😉