First, the Windows Update Assistant checks whether your system is fit for Windows 10 2004 at all. If so, the download of the new Windows version starts in the background. Microsoft then installs a half-hour grace period. You could use this to close all open programs and possibly save some important data. As already mentioned above, you should definitely have made a backup in advance, because there can also be problems with upgrades. That doesn’t have to be the case with you, but it’s safe to be sure.