How to change DTD structure of a XML file

Hi everbody...wishing you a very happy new year....this is my first mail to
this group. I am trying to make an application (using java for coding)where
i need to changed the DTD structure of the XML file.  that is i want to
rearrange the data in the XML file. How can I do this . well i am very new
to the world of XML i will appreciate if someone could guide me and let me
know the best possible way to do this and some resources available on the
net which will be helpful for me.

Thanx,
Cheers,
Hitesh Dhingra