<x1>
<x2>
<x3>
<v>111</v>
<v>222</v>
<v>333</v>
</x3>
</x2>
</x1>

select 111, 'aaa' from dual union all
select 222, 'bbb' from dual union all
select 333, 'ccc' from dual

with a as
(select
xmltype('<x1>
<x2>
<x3>
<v>111</v>
<v>222</v>
<v>333</v>
</x3>
</x2>
</x1>') as xx
from dual
) 
select xx 
, 
XMLQuery(
'
copy $xx2 := $xx
        modify(
          (for $i in $xx2/x1/x2/x3/v 
           return replace value of node $i with  (if (data($i) = $xfrom) then $xto else data($i))
            )         
        )
        return $xx2
' 
PASSING a.xx AS "xx", '333' AS "xfrom", 'ccc' AS "xto" 
RETURNING CONTENT)
from a