The template data GUI editor does not currently support the editing of the 'maps' parameter.
About Template Data in general:
https://en.wikipedia.org/wiki/Wikipedia:TemplateData
There's a brief mention of how to get to the TemplateData Editor GUI here:
https://en.wikipedia.org/wiki/Wikipedia:TemplateData#Adding_TemplateData
More details on the TemplateData Editor:
https://www.mediawiki.org/wiki/Help:TemplateData#TemplateData_editor
Some references about 'maps' specifically, which is not editable using the TemplateData Editor:
- https://www.mediawiki.org/wiki/Citoid/Maps_TemplateData
- https://www.mediawiki.org/wiki/Extension:TemplateData#Maps_object
==Getting started==
# [[ https://www.mediawiki.org/wiki/How_to_become_a_MediaWiki_hacker | Install mediawiki ]]and the [[ https://www.mediawiki.org/wiki/Extension:TemplateData | templatedata extension]] locally, as well as the [[https://www.mediawiki.org/wiki/Extension:Scribunto | 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