I kept getting an error message that my internet connection failed when trying to download the update. I have a MacBook Air 2019 and this is what worked for me after trial and error:
Cleared cache and cookies in browser
Restarted in safe mode
While in safe mode, I deleted old files and apps that I don't use and this seemed to move the download along
I kept my Mac charging while it was downloading and installing
It took about 20-25 minutes to complete.
sunflower220
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