Hi @Fink,
In my experience (if you are in a testing phase) the only way I found to "reset" was to delete the metric and redo the integration.
I would not recommend that for the reason that I don't know if you already have active flows that rely on that metric, in this case "Placed order".
I would recommend that you set the viewing period excluding the days prior to Go Live.
Here is another similar question that had been asked: https://community.klaviyo.com/campaigns-and-flows-30/how-to-reset-metric-dimensions-6816
I hope I was able to help you out!