from lxml import objectify, etree
root = etree.fromstring(\'\'\'
You could try this:
r = etree.fromstring('...')
element = r.find('//avp[@name="Host-IP-Address"]')
# Access value
print 'Current value is:', element.get('value')
# change value
element.set('value', 'newvalue')
Also, note that in your example you're using the text() method, but that's not what you want: the "text" of an element is what is enclosed by the element. For example, given this:
this is the text
The value of the text() method on the element is "this is the text".