How to replace XML node in the CLOB column data in oracle

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Oracle

Dear experts,
I have CLOB column in the table and which is having data like below:

<billingDetails>
<createDate>2021-01-10T16:01:22.000-07:00</createDate>
<dueDate>2021-02-10T16:01:22.000-07:00</dueDate>
<fromDate>2021-02-12T16:01:22.000-07:00</fromDate>
<toDate>2021-03-12T16:01:22.000-07:00</toDate>
<mailed>false</mailed>
<autopay>true</autopay>
<reason><=$0 balance></reason>
</billingDetails>

I would like to replace node

<reason><=$0 balance></reason>

into

<reason>not valid</reason>

Means updated xml should be like below

<billingDetails>
<createDate>2021-01-10T16:01:22.000-07:00</createDate>
<dueDate>2021-02-10T16:01:22.000-07:00</dueDate>
<fromDate>2021-02-12T16:01:22.000-07:00</fromDate>
<toDate>2021-03-12T16:01:22.000-07:00</toDate>
<mailed>false</mailed>
<autopay>true</autopay>
<reason>not valid</reason>
</billingDetails>

What I have tried:

I have used below query but getting exception 
<pre>UPDATE tab1 SET clob_column =
   UPDATEXML(clob_column,
   '/billingDetails/reason/text()','not valid')
   WHERE sid =1

Exception:

[Error] Execution (20: 4): ORA-00932: inconsistent datatypes: expected - got -

Please help me to resolve and update xml node?
Thanks in advance