Microsoft has a nice article telling you all about preconfiguredEntries here but neglect to point out that while developing if you change default properties it won’t be reflected in the web part even after a rebuild.
It’s simple enough to fix when changing the properties:
- Stop your gulp serve from running.
- In your terminal run gulp clean.
- Then run gulp serve --nobrowser again to rebuild, you will then see that the new default properties have taken effect!