How do I install macOS Big Sur on my MacBook Air?

If this is a 2014 MacBook Air with 4 GB RAM and older CPU/GPU, then you don't want to put Big Sur on it as it will be almost unusable with reduced performance. I know this from personal experience and my Air now has Catalina on it — which isn't much of a performance gain from Big Sur. Big operating systems and a small RAM footprint are not recommended.

From Apple's technical specifications:

* If upgrading from macOS Sierra or later, macOS Big Sur requires 35.5GB of available storage to upgrade. If upgrading from an earlier release, macOS Big Sur requires up to 44.5GB of available storage. 

Your available space is pretty minimal even for upgrading from a later version of macOS and not sufficient for upgrading from earlier versions. You'll need to clear some space.

BTW, for future issues you may need help with, a more complete question will help us help you. This may help you craft a good query:

Writing an effective Apple Support Commun… - Apple Community

Regards.