How do you select the DTD for a XML file?

Hi,

First time I have opened a XML file I was asked to browse  for a XML Schema or a DTD.
I selected a DTD, but it was the wrong DTD file.

Now I want to select another DTD file for my XML file. But I can't find any option to select a new DTD, and every time I open a XML file it has the first selected DTD, which is wrong!

How can I select a new DTD for my XML files?

Thank you