1. Create Big Sur Installation USB:
– From a Macbook that already have Big Sur installed, download Big Sur from Apple Store.
– Run this command to setup an install media (USB):
sudo /Applications/Install\ macOS\ Big\ Sur.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume
Change MyVolume with the name of USB drive
2. Install new MacOS from USB:
– Press and hold the Option (Alt) key immediately after turning on or restarting your Mac.
– Select the volume containing the bootable installer. Then click the up arrow to install.
– Maybe use Disk Utility to erase the old Hard drive of the Macbook first.
TNOnline 