I have this xml value in a CLOB column in Oracle 11g:
You have a namespace in your top-level Energy node, so you aren't matching without; the UPDATEXML documentation shows you can optionally supply a namespace string.
So you can do this, using your example data:
create table tmp_tab_noemail_test (sce_msg clob);
insert into tmp_tab_noemail_test values (
'<Energy xmlns="http://euroconsumers.org/notifications/2009/01/notification">
<Gender>M</Gender>
<FirstName>MAR</FirstName>
<Name>VAN HALL</Name>
<Email/><Telephone>000000000</Telephone>
<InsertDate>2013-10-09</InsertDate>
</Energy>');
update tmp_tab_noemail_test p1
set p1.sce_msg = updatexml(xmltype(p1.sce_msg),
'/Energy/InsertDate/text()','Not Valid',
'xmlns="http://euroconsumers.org/notifications/2009/01/notification"').getClobVal();
After which you end up with:
select sce_msg from tmp_tab_noemail_test;
SCE_MSG
--------------------------------------------------------------------------------
<Energy xmlns="http://euroconsumers.org/notifications/2009/01/notification"><Gender>M</Gender><FirstName>MAR</FirstName><Name>VAN HALL</Name><Email/><Telephone>000000000</Telephone><InsertDate>Not Valid</InsertDate></Energy>
Or with slightly less scrolling:
select XMLQuery('//*:InsertDate' passing XMLType(sce_msg) returning content) as insertdate
from tmp_tab_noemail_test;
INSERTDATE
--------------------------------------------------------------------------------
<InsertDate xmlns="http://euroconsumers.org/notifications/2009/01/notification">Not Valid</InsertDate>
You could also wildcard the update:
update tmp_tab_noemail_test p1
set p1.sce_msg = updatexml(xmltype(p1.sce_msg),
'/*:Energy/*:InsertDate/text()','Not Valid').getClobVal();
... but it's probably better to specify the namespace.