MacBook Air Storage space

You are going to be very hard pressed to clear enough space to allow Big Sur to install. Have a look over Here for ideas Also, check if all your installed Apps are 64 bit or 32 bit. Big Sur ONLY supports 64 bit apps. Clear out the 32 bit apps and some of the unneeded e-mail and maybe, just maybe space requirements will be met.

Followup - have a read over Here as to what is involved. Understand, everything, macOS, Apps, media, contacts, pics etc will be lost except for that which is kept on iCloud.

Do keep us updated success or failure or additional issues. We are here to assist.

Thank you for image of your situation re: Empty Space available. The New OS 11 sees 13.95 GB empty space right now and asking for Addition 20.06 GB so that means minimum of 35 GB needs to be available before it can proceed. Do you think you can achieve this?

If not, one could go an Apple Store and wipe the drive and install the Newest macOS. OR, if you are certain that loosing everything is the route, it can be done by you.

Congratulation and really my pleasure to help. Enjoy the New and Improved MacOS 11