modifying elements value

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>)

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

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

Thanks
Ajit