Update MacBook Air

One might be better served upgrading to High Sierra first as it will better position the computer for the eventual move to Big Sur. The upgrade to High Sierra should convert the Internal Drive to the new APFS filing system needed in Big Sur. Plus give you time to access getting newer application that are 64 Bit as Big Sur ONLY Supports everything 64 Bit. There is not workaround for anything 32 bit. Mojave was the last to support 32 bit.

With the above information in-hand - follow this How to get old versions of macOS and verify what version this computer Qualifies to install. Make sure to use Only Safari to commence the download as Others may not work

Make a Time Machine Backup Before attempting. Notation - Mojave is the Last version to support 32 bit applications / extensions and Drivers. Catalina and Big Sur Only support 64 bit and there is no workaround.

Are there any AntiVirus, Disk Cleaner, Optimizers, Un-installers, etc installed which should be removed as per Developers Instructions. They are useless, unneeded, cause havoc and interfere with the normal operation of the OS and may even Corrupt the OS requiring a Reinstallation

How to upgrade to macOS Big Sur - Apple Support

Welcome ;-))

When ready for next step to Big Sur - Tips How to Upgrade to Big Sur  but before that attend to the High Sierra upgrade

Open the App Store. Search for Big Sur.

FWIW, a MacBook Air 2015 can run Big Sur. I would have said it couldn't if that were the case.

I should have mentioned, it's unnecessary to upgrade to High Sierra first. Will just take twice as long.

Proceed in what every direction you see as best for your needs and comfort level.