Q. Windows 10 1803 update gives this message  0xc1900101. Then windows update starts the download again.

Then it’s message 0x80070005.

How to escape from this.Win10 pro, 1709

*Modified title for accuracy*

*Original title: Windows 10 1803 update fails with message  0xc1900101.*

Answer.

0xC1900101 is generic error code and points to driver incompatibility while you opt to upgrade. You need to read the dialog, which likely came before this error message. There should be additional information with the detail. The dialog will tell you what devices are having a problem. That is the one you need to get a new driver for. 

Once you fixed the driver(s) mentioned with detail, the error will no longer persist.