How can I upgrade my macbook air? I don't have enough space

See if these suggestions given in below posted Apple Support articles help.

Storage

• Other: Contains files that don’t fall into the categories listed here. This category primarily includes files and data used by the system, such as log files, caches, VM files, and other runtime system resources. Also included are temporary files, fonts, app support files, and plug-ins. You can't manage the contents of this category. The contents are managed by macOS, and the category varies in size depending on the current state of your Mac.
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       https://support.apple.com/guide/system-information/see-available-storage-space-syspf9b375b9/11.0/mac/11.0

      Delete files

       https://support.apple.com/guide/system-information/find-and-delete-files-syspf5a64aa6/11.0/mac/11.0

       Click the “Manage”button. Wait until Green “Completed” button appears.

       Optimize storage

       https://support.apple.com/guide/system-information/optimize-storage-space-sysp4ee93ca4/11.0/mac/11.0