Use text
property. For example:
html = '''...
<slide>
America
<a> 2 </a>
<b> 3 </b>
<c> 4 </c>
</slide>
<slide>
Germany
<a> 5 </a>
<b> 6 </b>
<c> 7 </c>
</slide>
....'''
import lxml.html
root = lxml.html.fromstring(html)
for slide in root.xpath('.//slide'):
slide.text = ''