copy and replace

How would i copy an xml completly ( attributes and elements ) and replace
just 1 value.
( see xml example where 100 needs to be replaced by 200 ).
I did manage to copy everything but can't replace the data-value (100 ->
200 ) of the Data tag

Thanks

XML

<Row>
        <Cell Index="6" StyleID="s21">
                <Data Type="String">100</Data>
        </Cell>
</Row>

Result XML

<Row>
        <Cell Index="6" StyleID="s21">
                <Data Type="String">200</Data>
        </Cell>
</Row>



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