KiCAD5.1.9: library paths cannot be set


I have just installed KiCAD5.1.9 on Windows10.
As I want to keep several KiCAD versions, I did not install to the default folder /kicad.

Now, I am having severe problems accessing the symbol and module libraries as the path substitutions point to the wrong folder, i.e. not to the installation folder.
To be more precise:
In the KiCAD main menu window I open Preferences-Manage Symbol Libraries.
There, on the bottom I see Path Substitutions, e.g. for ${KICAD_SYMBOL_DIR}.
The path next to it does NOT point to the installation folder \share… but to some no longer existent folder of a previous installation.
More severe: The path entry cannot be edited. I can change it by copy and paste, but the change is not memorized, i.e. if I close the window and reopen it, the original non-existent path is shown and not the updated one.

How can I correct this path???

Found it.
Preferences-Configure Paths