Microsoft revamped the Windows Update user interface in Windows 10 and Windows Server 2016 so that you can't control when the updates are installed anymore, only which (maximum of) 12 hour period the computer will not reboot during.
Instead, you can go into the Group Policy Editor and make this change.
- Open a blank MMC window. You can do this by running mmc.exe
- Add the Local Computer Policy snap-in to the window.
- Expand Local Computer Policy, Computer Configuration, Administrative Templates, Windows Components, Windows Update.
- Select Configure Automatic Updates and specify which day and time you want updates to download and install. Or just download.