Should I update my MacBook Air to Big Sur?

That is up to you. Assuming your device is new enough to be compatible with Big Sur, it is recommended to stay current with security updates. However, if you have particular apps/software that is critical for you and it is NOT compatible with Big Sur, you may want to consider waiting until you no longer need that or have found a reasonable substitute.

Big Sur requires a minimum of 35.5 to 45 GB of empty space depending on the OS version you are running now. In order to have a computer perform well, you should also always have an absolute minimum of 15 - 20 GB of empty space at all times. If you fill it up, it will simply stop functioning; the OS needs room to function properly.

If you too much stuff, offload it to an external drive or delete items no longer needed.