Microsoft embeds Flash Player in Internet Explorer and updates are distributed by Microsoft via Windows Updates. Alternatively to running Windows Update to find the update, you can download and install the update itself. See Microsoft security advisory: Update for vulnerabilities in Adobe Flash Player in Internet Explorer: May 12, 2015 to download the Internet Explorer update containing the May release of Flash Player (18.104.22.168).
Links to the updates are also posted at the bottom of the Flash Player Issues | Windows 8 page. Since the Microsoft KB article URLs (the one I link to first) change with each release, you may want to bookmark this Adobe help page page for future reference.
If you're looking for Flash Player for non-IE browser on Windows 8.1, launch that specific browser and navigate to https://get.adobe.com/flashplayer. There are three different installer files for Windows (depending on browser) and this page detects the OS and browser and then offers the installer for the OS/browser combination.