Windows 10 Anniversary update is now available to general public as Explained here and on the community forums I saw a large number of people reporting 0X80070057 error while updating to the latest build using the upgrade assistant.I will discuss the solution in detail in this article.
Generally error code 80070057 signals corruption of Windows update files (Source) but looks like this isn’t the case here. On one of my computers, I noticed that the upgrade assistant tool is throwing this error by checking in the log files. So to get the Anniversary update, use an Alternate called Media creation tool which is present on the same page of the Windows 10 Software download as shown:
After downloading the tool, accept the license terms and click upgrade this PC now, windows would automatically Install the Anniversary update.For a detailed explaination see this article:
Personally I find MCT more reliable than the upgrade assistant as well as it also allows saving the Installation file if you want.
Questions? Feel free to comment!