There is a hidden file in your user account ....... /home/user/.kde (note the period before kde indicating a hidden file).
You can rename that file -- say to .kde-old -- and at next login the default settings will be recreated for that user.
If you wish to change back then reverse the process.
If you wish to discard everything in .kde-old you can delete it.
There may be files within that folder that you wish to keep so I would caution against deleting it so that you might recover them if required.