For replace the "name" word with the "something" word, use:

 sed "s/\(<username><\!\[[A-Z]*\[\)name\]/\1something/g" file.xml

That is going to replace all the occurrences of the specified word.

So far all is outputted to standard output, you can use:

 sed "s/\(<username><\!\[[A-Z]*\[\)name\]/\1something/g" file.xml > anotherfile.xml

to save the changes to another file.