There's technically no way to modify the summary of an existing event through the JSON API. This is because the JSON API is working with "Event Summaries" in the first place which don't actually have a summary field at all.
An "EventSummary" is actually the rollup of multiple events or occurrences that represents one row on the event console. The EventSummary contains information such as uuid, first seen time, last seen time, and etc. It is the most recent occurrence where that summary is kept.
So the easiest way to update the summary that you see is to send another event with the new summary. If you can assign the same eventKey to the original event and the one you create they can be deduplicated and your summary will replace the last.