macOS high sierra
hi my MacBook air have macOS high sierra version 10.13.6 and i can't upgrade to the new version
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It depending on the Age ( Year ) of the computer. For those computers that do not qualify for Big Sur ( Latest macOS ) one can download and install older macOS Version Here. For computers that do qualify for Big Sur - read over here.
Note: moving from High Sierra to Mojave is straight forwards. Moving from HS to Catalina - Does Not Support any 32 Bit Applications / Extensions. From HS directly to Big Sur is much more involved in that the Empty Space Requirements for a successful install are very important Before attempting this move. And, like Catalina, Big Sur does not support any 32 bit Apps / Extensions.
Lastly, any time before doing a macOS UpGrade - a Time Machine Backup is really necessary. I always suggest doing two TESTED Clones each to separate external drives before an UpGrade - insurance if ate upgrade goes sideways.