Right now, if you try editing a relationship, you can lookup a new entity, but you can't change the URL, e.g. http://musicbrainz.org/edit/relationship/edit?returnto=http%3A%2F%2Fmusicbrainz.org%2Fartist%2F7e84f845-ac16-41fe-9ff8-df12eb32af55%2Frelationships&type1=url&type0=artist&id=287960
The only way to change the URL is to edit the URL itself - which then changes all relationships using that URL. If you want to only change one relationship, all you can do is remove the relationship and add a new one. It would be a lot nicer if we could just edit the relationship instead.