my mac book air that has sierra on it.

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

"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.