If you want to update an existing metadata key, you need to use the POST method and the same, three-part URL path, /images/id/metadata.
The id part is the numeric id of the server image. For example, if you wanted to set the owner metadata key of the server image whose id is 1, the path would be /images/1/metadata.
If you were to use curl to set metadata keys, this is how you'd do it:
$ curl -X POST -H "X-Auth-Token:999888777666" -H "Accept: application/json" -d '{"metadata":{"owner":"adam"}}' http://localhost:8774/v1.1/openstack/images/1/metadata
