The theme option will not lost, for safe, the backup is necessary.
If you add the CSS into child theme, the added CSS will not lost.
You can not upload the same theme before delete it, because it will be conflict, they are same fold name.
For safe, you could rename the old Air theme folder on FTP firstly.
So the correct order should be:
1) backup site on Siteground (my host(
2) download latest Air theme version from Themeforest
3) rename old Air theme folder(like:old-air) on FTP
4) upload new theme fold to WordPress