At a basic level choosing to upgrade to the next build in Windows 10 Tech Preview is almost like an upgrade to a new os. This leaves behind such folders as windows.old which usually contain files that were used in your previous version of Windows. If like me you use an SSD then every GB is precious and so once your sure your system is as it should be you can always reclaim this disk space back by running Disk clean. I prefer to use a slightly different version which is a lot more thorough. Open an admin command prompt and type: cleanmgr/sageset:1 Press enter and a Disk clean set up box will appear but this time their are many more check boxes than the original and one does have to careful on what you choose. Probably the worst outcome is that you may not be able to uninstall a certain update so be careful what you pick. Once you've made your choices click ok and the box will disappear. To actually run the Disk clean app you've now created you need to type in a admin command prompt: cleanmgr/sagerun:1 Press enter and the new Disk Clean app will cycle.. You don't have to set it up again and can run the app whenever you type the last command.