my mac book air that has sierra on it.

"my mac book air that has sierra on it.: how do i get Mojave for my mac book air that has sierra on it?"

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 it is Mentioned here: macOS Monterey is Compatible with these Computers - Apple Support. If it is not mentioned in that link, then installing it is just not an option. :(

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.

That depend on wether your Macbook Air can support Mojave. See here supported models: Link-> macOS Mojave - Technical Specifications

If your MacBook Air is one of the supported models, then see here to update to Mojave: link-> How to get old versions of macOS - Apple Support