Update existing alert in grafana using API

[jangaraj]

Use browser console:

Is there a supported way to access the dashboard json and upload a changed version?

Use Grafana dashboard API. BTW: UI uses API as well, so you can use browser console to watch API requests from UI in real life - you will have idea about API endpoints and payloads without reading Grafana API doc. There is also swagger UI, which may help you to develop API queries.