how to modify planned order using rest API

Summary:

We are loading Planned Orders through REST API in a Supply plan. Each day before executing these loads we want to update the ones that are in Firm status to unfirm status and do not have a Purchase Order assigned to them.

Does anyone know how we can perform this deletion or modification? We did not find the API to do it but it seems strange that we can modify by screen and not by API.

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