Most people don't have a problem with Windows Update and it works fine, downloading essential updates in the background and automatically installing them when you shut down or restart.
Sometimes though, Windows Update gets in a muddle and it may not download one or more of the updates offered, it might stop part way through downloading, it may fail to install one or more updates, or it can even repeatedly ask you to install an update you have installed before.
Fortunately, there is a solution.
If there is a problem downloading an update, you could try downloading the latest service pack to the disk drive and then run it from disk to install it.
There may be updates after the service pack was created though, but it's worth a try. Download Windows service packs from the Service Pack and Updates Center.
Microsoft has several automated tools on the website that can solve many of the most common Windows Update problems. For example, go to Windows Update Troubleshooter and download the troubleshooter for your version of Windows.
The Automated Troubleshooter should be all you need.
There is one more useful tool at the Microsoft website and this is a page that enables you to reset the Windows Update components.
There are sections for Windows 10, and Windows 8.1, 8 and 7. There used to be online versions you could run, but now they download the troubleshooters. Hopefully, resetting Windows Update will solve the problems you are having.