Indeed, if you simply click the folder and then press the Delete key, Windows will tell you you need permission, yada-yada, etc. Here's the proper way to delete the Windows.old folder: Step 1: Click in Windows' search field, type Cleanup, then click Disk Cleanup.Step 2: Click the Clean up system files button.creenshot by Rick Broida/CNET. Second, unless you're seriously strapped for space on your hard drive, you don't have to do anything: Windows 10 will automatically delete the Windows.old folder one month after you performed your upgrade.