Maximized game in window mode, how to resume normal size?

So the following problem: I have a computer game installed, but which had strong tearing in normal full screen view, but not in the window view.
Now I was so smart googling how to maximize this "window" in the hope that I could now play in full screen mode without tearing.
Then I came to the glorious idea to maximize this window on the task manager, which worked well, but the game is now unplayable slow running, about 10 fps.
In the hope that I somehow get this back, so the "window" again small, I have already restarted the laptop and the game again uninstalled and reinstalled, but it opens again with maximized window…
Do you have any idea how to solve this problem?
And why did it save this stupid attitude over Steam?

"Alt" and enter would have to work.

We have the same profile picture!

Well, with ALT + Enter I switch between full screen and window mode, unfortunately I now have a maximized window mode, so change only between two full screen modes, both of which run badly…

So. This is difficult.
Delete the local data (uninstall), then search your PC for related data and delete all. (Many games also save configuration data.) In AppData "Roaming or Local")

If that does not work, I would recommend contacting the game developer or Steam support.