It is currently not feasible to download Windows 12 for free because it has not yet been published. But if its release schedule is similar to Windows 11, then its users as well as maybe those of Windows 10 will be able to upgrade for free if they already own a working copy of Windows. You may get Windows 12 from the Microsoft website once it becomes available. In addition, You can use a DVD or USB flash drive to build your own installation media.