How to see changes in data source plugin

Hi i used the " Simple JSON Datasource - a generic backend datasource" from https://github.com/grafana/simple-json-datasource

but when i am doing any changes in this plugin, it is not getting reflected.

i have run “npm run build” command for that which is giving me this error “’.’ is not recognized as an internal or external command,”

Please some et me know how to do the chnages in custom datasource plugin

Thanks