You are looking at documentation for an older release. Not what you want? See the current release documentation.
In the previous section, you get a file using file service and the file's ID.
You can modify your local copy of the file and upload it back to the Content drive using the file service again but with PUT method.
curl -T test.txt -X PUT -H "Content-Type:text/plain; charset=UTF-8" -u root:gtn http://localhost:8080/rest/private/cmisatom/Managed%20Sites/file/03dd409c7f000001010b7e050789d358
Then check the new content of /intranet/documents/test.txt in Sites Explorer.