want to upgrade to big sur from Macbook 12 2015

Big Sur is the highest possible. See the following from Identify your MacBook model - Apple Support

MacBook (Retina, 12-inch, Early 2015)

Colors: Space gray, gold, silver

Model Identifier: MacBook8,1

Part Numbers: MF855xx/A, MF865xx/A, MJY32xx/A, MJY42xx/A, MK4M2xx/A, MK4N2xx/A

Newest compatible operating system: macOS Big Sur

To upgrade, see instructions in How to download macOS - Apple Support

jtokerud Said:

"want to upgrade to big sur from Macbook 12 2015: I want to upgrade to MacOS Big Sur from El Capitan."

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For reference...

Updating the macOS:

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:

I 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:

See if the Model you found is shown here:

• 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
• OS X El Capitan 10.11 - Hardware Requirements - Apple Support
• OS X Yosemite 10.10 - 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.

Before upgrading, you may want to check what 32-bit apps you have on your system.

El Capitan will run 32-bit applications. Catalina and later will not.

Select About This Mac from the Apple () menu. Click on the System Report… button, then on the Applications heading in the sidebar. Wait for the Mac to build the applications list. Click on the 64-bit (Intel) column header to sort by that field (so all the apps with "No" are grouped together for easy identification).

This will tell you which applications you need to update (if possible), replace, or stop using.

Check if you have 32-bit apps. macOS Mojave or earlier work with 32 bit 10.15 or newer will not run.

On 32 bit apps.