Windows 10 is great-ish. But sometimes you want to keep using the OS you’re on, be it Windows 7 (fine) or one of the Windows 8 variants (what’s wrong with you?!). To hide the tray icon that keeps wanting you to upgrade, run these two PowerShell commands (assuming you’ve never tweaked this setting before. Error messages are probably fine):
New-Item HKLM:\Software\Policies\Microsoft\Windows\Gwx New-ItemProperty HKLM:\SOFTWARE\Policies\Microsoft\Windows\Gwx -Name DisableGwx -PropertyType DWORD -Value 1