for $d in Root("/PersonDB/Person") 
let $newdoc :=  
   update $d using replace value $d/Person/City/text() with "Vancouver" 
where $d/Person/City = "Paris" 
return $newdoc 

  <Person Id = '1' Active = 'true'>
    <!--firstname comment-->
    <?proc instrn ?>
    <FirstName>
      John
    </FirstName>
    <LastName>
      Doe
    </LastName>
    <City>
      Vancouver
    </City>
    <BirthDate>
      1962-03-24T05:00:00
    </BirthDate>
    <Age>
      27
    </Age>
    <StockValue>
      826.8
    </StockValue>
  </Person>
  <Person Id = '11' Active = 'true'>
    <!--firstname comment-->
    <?proc instrn ?>
    <FirstName>
      Frank
    </FirstName>
    <LastName>
      Crowe
    </LastName>
    <City>
      Vancouver
    </City>
    <BirthDate>
      1960-05-30T05:00:00
    </BirthDate>
    <Age>
      67
    </Age>
    <StockValue>
      826.8
    </StockValue>
  </Person>
  <Person Id = '21' Active = 'true'>
    <!--firstname comment-->
    <?proc instrn ?>
    <FirstName>
      George
    </FirstName>
    <LastName>
      So
    </LastName>
    <City>
      Vancouver
    </City>
    <BirthDate>
      1990-05-19T05:00:00
    </BirthDate>
    <Age>
      27
    </Age>
    <StockValue>
      826.8
    </StockValue>
  </Person>
  <Person Id = '31' Active = 'true'>
    <!--firstname comment-->
    <?proc instrn ?>
    <FirstName>
      Russ
    </FirstName>
    <LastName>
      Quan
    </LastName>
    <City>
      Vancouver
    </City>
    <BirthDate>
      2000-01-01T05:00:00
    </BirthDate>
    <Age>
      67
    </Age>
    <StockValue>
      826.8
    </StockValue>
  </Person>
  <Person Id = '41' Active = 'true'>
    <!--firstname comment-->
    <?proc instrn ?>
    <FirstName>
      Paul
    </FirstName>
    <LastName>
      Burnnet
    </LastName>
    <City>
      Vancouver
    </City>
    <BirthDate>
      2001-03-07T05:00:00
    </BirthDate>
    <Age>
      27
    </Age>
    <StockValue>
      826.8
    </StockValue>
  </Person>
  <Person Id = '51' Active = 'true'>
    <!--firstname comment-->
    <?proc instrn ?>
    <FirstName>
      Leif
    </FirstName>
    <LastName>
      MacDonald
    </LastName>
    <City>
      Vancouver
    </City>
    <BirthDate>
      1937-06-02T05:00:00
    </BirthDate>
    <Age>
      67
    </Age>
    <StockValue>
      826.8
    </StockValue>
  </Person>
  <Person Id = '61' Active = 'true'>
    <!--firstname comment-->
    <?proc instrn ?>
    <FirstName>
      Michael
    </FirstName>
    <LastName>
      McConnel
    </LastName>
    <City>
      Vancouver
    </City>
    <BirthDate>
      1994-06-11T05:00:00
    </BirthDate>
    <Age>
      27
    </Age>
    <StockValue>
      826.8
    </StockValue>
  </Person>
  <Person Id = '71' Active = 'true'>
    <!--firstname comment-->
    <?proc instrn ?>
    <FirstName>
      Jim
    </FirstName>
    <LastName>
      Day
    </LastName>
    <City>
      Vancouver
    </City>
    <BirthDate>
      1983-02-28T05:00:00
    </BirthDate>
    <Age>
      67
    </Age>
    <StockValue>
      826.8
    </StockValue>
  </Person>
  <Person Id = '81' Active = 'true'>
    <!--firstname comment-->
    <?proc instrn ?>
    <FirstName>
      Peter
    </FirstName>
    <LastName>
      Marsh
    </LastName>
    <City>
      Vancouver
    </City>
    <BirthDate>
      1929-01-01T05:00:00
    </BirthDate>
    <Age>
      27
    </Age>
    <StockValue>
      826.8
    </StockValue>
  </Person>
  <Person Id = '91' Active = 'true'>
    <!--firstname comment-->
    <?proc instrn ?>
    <FirstName>
      Gwendal
    </FirstName>
    <LastName>
      Thursk
    </LastName>
    <City>
      Vancouver
    </City>
    <BirthDate>
      1999-09-26T05:00:00
    </BirthDate>
    <Age>
      67
    </Age>
    <StockValue>
      826.8
    </StockValue>
  </Person>