You can update an existing adgroup's creative by creating a new adcreative and then update the adgroup with the new adcreative id:
Create a new ad creative with your url_tags (in this example, "key=val"):
curl \ -F "type=27" \ -F "object_id=object_id" \ -F "story_id=story_id" \ -F "url_tags=key=val" \ -F "access_token=_" \ "https://graph.facebook.com/act_12134567/adcreatives"
Update your adgroup with the new adcreative id:
curl \ -F "creative={'creative_id':new_adcreative_id}" \ -F "access_token=_" \ "https://graph.facebook.com/adgroup_id"