Oops... it looks like this feature slipped into our production environment a little too soon. In our next update to Tr.im (in the next couple of weeks) we will be including some new features, one of those is the ability to Edit an existing link which is what you are seeing here.
This will allow you to change the custom key word on any existing tr.immed links you have by clicking the Edit icon. Of course at the moment that is not going to work as it should but you can expect it to be able to do that in a couple of weeks once we release our next update!
Thanks for pointing out this to us! If you have any other feedback about Tr.im then we'd love to hear it!!