Since June 2015, Windows is showing a little Windows icon in system notification area on the right. It suggests to get a free upgrade to Windows 10.

However, there seems to be no easy way to hide that icon. However, you can hide it, in some way at least.

First, right click on the notification area and choose properties.

Then, the control panel will open. At the bottom of the windows, choose Customise notification icons.

Finally, look for the icon with the name GWX and that white Windows icon. However, make sure you uncheck Always show all icons and notificatoins on the taskbar. Now for the GWX icon, in the drop down menu pick Hide icon and notifications. Finally, click ok to confirm.

Another option is, arguably, easier. Rename this folder:

C:\Windows\system32\GWX

To something else, like:

C:\Windows\system32\GWXisGone

Succes! :)