I've tried downloading and upgrading multiple times to Big Sur from Catalina, keep getting error message "This copy of the Install macOS Big Sur.app application is damaged, and can't be used to install macOS." Have sent that file to trash and tried redownloading, get the same result. Help?
MacBook Pro (2020 and later)
After you trash the file and empty the trash, reboot the Mac, something may still be mounted and in use. On Catalina open Terminal and try the following command to download Big Sur.
softwareupdate --fetch-full-installer --fetch-installer-version 11.5.2
When it finishes downloading:
/Applications/Install\ macOS\ Big\ Sur/Contents/Resources/startosinstall --agreetolicense --nointeraction
Alternative:
Create a bootable USB flash disk using at least a 16GB+ flash drive. This will create an external USB drive you can boot from to install Big Sur. Per this Apple KB https://support.apple.com/en-us/HT201372
Format the USB flash drive in Disk Utility using HFS or FAT32 and name it "MyVolume" case sensitive so the next command will work.
sudo /Applications/Install\ macOS\ Big\ Sur.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume
On a T2 equipped Mac you will need to boot into recovery mode and the Utilities menu and Security / Startup Options and allow USB booting. Then you can reboot and hold Option key on Intel Mac's to boot from the flash drive installer.
macOS Big Sur.app application is damaged