When you log out:
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If you are using Gnome’s login screen (GDM), there will be an options button in the bottom right corner. Click it and you will be able to change between desktop environments.
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If you are using LightDM, there will be an icon of the desktop environment you are using (in your case KDE’s) over the password field. Click it and you will be able to change between desktop environments.
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If you are using KDE’s login screen, there will be the name of your desktop environment in the bottom left corner, click it and you will be able to change between desktop environments.
There are some other ones, too, but these are the ones I know.