I have MacBook Air 2019 and get told I don't have enough space to upgrade to Big Sur. Please help.
that's it that's the question.
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See the following from How to upgrade to macOS Big Sur - Apple Support
If upgrading from macOS Sierra or later, macOS Big Sur requires 35.5GB of available storage to upgrade. If upgrading from an earlier release, macOS Big Sur requires up to 44.5GB of available storage.
If you don't have enough available, the following may help: How to free up storage space on your Mac - Apple Support