This is very annoying and confusing, I know several people that installed Windows 10 by mistake on old hardware and are now having trouble using their PCs. The following information is based on Windows 7, in previous versions of Windows the screens are different but its the same principle.
The Install Windows 10 app comes in as an Optional Windows update. If you are like me and prefer to chose whats installed in your PC you should still have it pending, open up the Windows update app (Control Panel / Windows Update), click on the Optional tab and look for KB3035583, click on it, its description at the right pane wrongly says: Install this update to resolve issues in Windows.
This is the Control Panel in View by: Small icons mode:
Clicking in Windows update will show you the currently available updates:
The important updates are selected by default, if you click the nn important updates are available phrase you can chose them:
These are the Windows Update settings that I use:
So, if you still don’t have the Install Windows 10 message you are probably going to find KB3035583 in your Optional updates, right click on it and chose Hide update, this will disable it from being selected for install.
If you already have the message then you are doomed!
No, just kidding… You can use one of several ways to get rid of it, either by just hiding the notification or by uninstalling the update. Uninstalling an update is easy if you know its name, just go to Control Panel / Uninstall a program:
You’ll get to the installed updates list, look for it and chose Uninstall. In the following example I just chose a random update to show you the Uninstall option. You can uninstall either by right clicking on the update or by selecting the update and then clicking the Uninstall button on the gray bar next to Organize button:
If you like the command prompt you can also open it up with Administrator privileges (right click the command prompt icon and chose Run as administrator) and give the following command:
WUSA /UNINSTALL /KB:3035583
There is also an Open Source app that can do it for you called I Don’t Want Windows 10, just download it and run it.