Go back to an earlier build will uninstall the most recent Windows 10 update to your PC.There's a time limit on this option: You can only go back to an earlier build within 10 days of updating.
In an obvious effort to minimize the Control Panel (or perhaps do away with it altogether) in Windows 10, Microsoft has removed Windows Update from the Control Panel and placed it in the new Settings menu.
Windows Update does not know if or when those files will be available to use, so your system requires a reboot to avoid any conflicts.
For security-related updates, your computer is unsafe before that reboot occurs: it is vulnerable to attacks which exploit the issue(s) fixed by the update requiring a reboot.
The Windows Update section is where you'll find (almost) everything you need to manage your Windows 10 updates.
You won't have to visit this section to manually download or install updates -- Windows 10 does all that automatically -- but you can come here to see your update history, change active hours, opt to schedule your restarts for a specific time, and choose how your updates are installed.