How to erase data from Macintosh HD - Data - Data? I just realised that there're over 400gb usage of data and I think there's that much data in my computer.
I need to an upgrade and I want to erase that from my Mac.
But I Cann't. Does anyone know how to?
Apple shows how to completely erase the disk here.

https://support.apple.com/en-us/HT208496

You have to boot to internet recovery since the system can’t erase itself while running, but you can sometimes script a reboot that then performs the erase...

https://github.com/grahampugh/erase-install

Or connect to a different Mac over target disk mode (or even a PC could format the disk and you would need internet recovery or external installer to reinstall the OS).