This might help:
PUT /caisd-rest/in/426144 HTTP/1.1
Host: SDMHost.ca.com:8050
X-AccessKey: 1962149949
Accept: application/xml
X-Obj-Attrs: ref_num, priority,description
Content-Type: application/xml
Cache-Control: no-cache
Postman-Token: fd7d7d40-9911-5cf3-71e5-7d8b8d7691d3
<in>
<summary>updating summary via REST twice</summary>
<parent id="426149"/>
</in>
Basically its updating incident id 426144 to have a parent id of 426149
_R