The template data GUI editor does not currently support the editing of the 'maps' parameter.
About Template Data in general:
There's a brief mention of how to get to the TemplateData Editor GUI here:
More details on the TemplateData Editor:
Some references about 'maps' specifically, which is not editable using the TemplateData Editor:
- Install mediawiki and the templatedata extension locally, as well as the Scribunto extension.
- Add some templates to your mediawiki extension - https://en.wikipedia.org/wiki/Template:Cite_journal is a good example to start with. You can export pages from wikipedia and import them into your own mediawiki installation using https://en.wikipedia.org/wiki/Special:Export and then http://dev.wiki.local.wmftest.net:8080/wiki/Special:Import
- Try editing template data both manually (in source editing mode) and with the GUI
I think the easiest way to understand 'maps' it is to see it in action; one way to do this is to install Citoid, which is the main reason we created 'maps' (but it is used by other tools now too, like proveit). An alternative to installing citoid locally is you can try it out on test.wikipedia.org; this is kind of sandbox where you can mess around without installing anything locally. I.e. try using visual editor to cite "http://www.example.com", and then go and edit the template data on https://test.wikipedia.org/wiki/Template:Cite_web and see if you can change the resulting citation.
Checkout tasks labelled good first task