你必须循环进行，一次一个节点.

You have to do it in a loop, one node at a time.

declare @MyTable table(Id int, MyXMLField xml)
insert into @MyTable values
(1, '<Parent1>
       <Parent2>
         <Child>test 1</Child>
       </Parent2>
       <Parent2>
         <Child>test 2</Child>
       </Parent2>
     </Parent1>')

declare @replacement nvarchar(4000) = 'something 1'
declare @Parent2Count int
select @Parent2Count = MyXMLField.value('count(/Parent1/Parent2)', 'int')
from @MyTable
where Id = 1

declare @Node int
set @Node = 1

while @Node <= @Parent2Count
begin
  update @MyTable set
    MyXMLField.modify('replace value of 
                          (/Parent1/Parent2[sql:variable("@Node")]/Child/text())[1] 
                          with sql:variable("@replacement")')
  where Id = 1    

  set @Node = @Node + 1
end