To export a process definition model to a BPMN 2.0 XML file:
GET api/enterprise/models/{processModelId}/bpmn20
For a previous version of the model:
GET api/enterprise/models/{processModelId}/history/{processModelHistoryId}/bpmn20
To import a BPMN 2.0 xml file:
POST api/enterprise/process-models/import
With the BPMN 2.0 XML file in the body as a multipart file and the file as value for the file property.