java- processing xml and modifying it to create new xml

[tyler1234]

In a java app that I am working on, there is a need to take an existing XML, then make changes to that xml and give the original as well as new xml, as output.Which parser do you recommend, such that it uses least memory? Where I can do the following--(a) easily take one portion of the xml, and insert it into another place in the same XML?(b) change some attributes in a section of the xml.