Windows 8.1 Update is a mandatory update for everyone running Windows 8.1
Windows 8.1 Update was officially launched on April 8 and although plenty of users have experienced issues when trying to deploy the new release, Microsoft decided to make it mandatory for everyone running Windows 8.1.The company explained that Windows 8.1 devices must be running Windows 8.1 Update by May 13; otherwise, they won't receive any other improvements and security fixes via Windows Update during the company's monthly Patch Tuesday rollouts.
Microsoft says that it had to turn down such a decision because all future changes will be based on Windows 8.1 Update, so if you're not running it when the company debuts these improvements, your device might not be able to get it.
Basically, on May 13, if your computer wasn't already upgraded to Windows 8.1 Update, the only thing that you shall see is the patch that allows you to deploy this OS version, and only then get access to the monthly fixes released as part of Patch Tuesday.
The problem, however, is that many users who actually tried to deploy Windows 8.1 Update ended up getting some errors that couldn't be bypassed, so in some cases, installing this new OS release isn't entirely users' choice. It remains to be seen if Microsoft manages to fix this for everyone by May 13.