How can i upgrade to Big Sur, I have 18 GB available in my MacBook Air

As @SeaPapp has outlined - must make much more Empty Space Available Before attempting the Big Sur upgrade.

Strongly suggest making a Time Machine Backup Before attempting ( after addition space is created ) and always recommend making two Tested Bootable Clones each to separate external Drives - Insurance incas it goes sideways.

Also, if the are any AntiVirus, Desk Cleaners, Optimizers, VPNs etc installed now - Remove them as per Developers Instructions, as they are useless, cause havoc, interfere with the normal operation of the OS and can even corrupt the OS. Good Article Here regarding the builtin Security of macOS

You'll first need to make more storage available on your Macbook Air as installation requires 35 Gb https://www.apple.com/macos/how-to-upgrade/