It is not impossible to do in Processing but I'm pretty sure it would involve quite a lot of work:
1. You are going to have to load the text of the XML file into Processing. Probably using loadStrings()
2. Set up some kind of personal menu or selection method for saying what kind of tag you are looking for. For example, if you selected <name> as the tag you want it would use that as the search string for step 3.
3. Search for your keyword string you picked from step 2. Then build a substring that goes until the ending tag (</name>). The substring would be the text between <name>"text you pulled"</name>.
4. Set up a method of writing a new text that will replace the substring you pulled. For example, if your pulled substring was "Mariel" and your new text was "Bob" it would place "Bob" between the <name></name>. This will require setting up some kind of typing method in Processing to write "Bob".
5. Use save strings to save your new text (the whole thing you originally loaded and modified) and save it with a .xml extension.