modifying elements value

Hello

I want to copy whole XML message as-is but want to modify only one
elements value to something else (in following example element <third>)

How can I do this using XSL

e.g.

<req>
        <first>123</first>
        <second>abc</second>
        <third>xyz</third>
        <forth>345</forth>
        <another_level>
                <element>12</element>
        <another_level>

</req>

to change to

<req>
        <first>123</first>
        <second>abc</second>
        <third>changed value</third>
        <forth>345</forth>
        <another_level>
                <element>12</element>
        <another_level>

</req>


Thanks
Ajit

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