Update XML in Oracle using XMLUPDATE

I have an XML document like this one and I have to update value of The time of begin by new text. How to do this with XMLUPDATE function, i.e. what is the XPath expression?

<dok>
    <dataSet>
        <foo>bla bla</foo>
        <field>START_DATE</field>
        <label>The time of begin</label>
    </dataSet>
    <dataSet>
        <bar>bla bla</bar>
        <label>The time of end</label>
        <field>END_DATE</field>
    </dataSet>
</dok>

My first idea was like this one:

UPDATE MY_TABLE SET 
XML_COLUMN = XMLUPDATE(XML_COLUMN, '/dok/dataSet/field/text()="START_DATE"/../label/text()', 'The date of start');

I don't like to use axis like following-sibling, because inside dataSet you may have various other elements in unknown order.

The rule shall be like: Update "label" at "dataSet" where "field" = 'START_DATE'