Thank you Rog131 for the advice,
I'm having same problem as vanderzwan and never found a solution on the internet!
Now I can change the click behaviour, that's great !!!

Can't they quit moving this around???

Rog131 wrote:Click Behavior

Bug 377310 - Single/double click option is inappropriately located in the mouse settings : https://bugs.kde.org/show_bug.cgi?id=377310
--> Single/Double Click Option at Workspace KCM: https://phabricator.kde.org/D12946

This patch adds the single/double click option to
the Workspace KCM.

With the Plasma 5.13 or later:

Thanks. It solves the problem that bothers me.

New location of the setting is in "Workspace behavior". I do find it dissatisfying that not only is a single click select / double click open the default option and that I have to set this each time but what angers me most of all is that the GUI developers don't seem to care about usability and keep moving the option on each release. Please spend some extra cycles on choosing a final and most refined location and do this all at once rather than making us hunt for the setting in forums on each release.

system settings -> workspace -> General Behavior:

https://pasteboard.co/IZZXoxq.png

Rog131 wrote:Click Behavior

Bug 377310 - Single/double click option is inappropriately located in the mouse settings : https://bugs.kde.org/show_bug.cgi?id=377310
--> Single/Double Click Option at Workspace KCM: https://phabricator.kde.org/D12946

This patch adds the single/double click option to
the Workspace KCM.

With the Plasma 5.13 or later:

Yes this works. Good advice. It was well hidden!