Update_Record.xml

<?xml version="1.0" ?>
<api method="put">
	<update module="Contacts">
		<record>
			<field name="email" value="@new_email"></field>
		</record>
		<return>
			<field name="id"></field>
		</return>
		<where>
			<field name="email" condition="eq" value="@old_email"></field>
		</where>
	</update>
</api>