To write results to a new file, you can use the write() function as follows:
import xml.etree.ElementTree as ET
for description in myroot.iter('description'):
new_desc = str(description.text)+'new text added'
description.text = str(new_desc)
Here, I have added some more text and an attribute to a tag named description that is already present in my file and writing the output to a new file.