我不知道 XML DML

使用替换可能对您有用.>

It might work for you to use replace instead.

update YourTable set
  XMLCol = replace(replace(cast(XMLCol as nvarchar(max)), 
                           '<MediaClass xmlns="MediaClass/1">', 
                           '<Book xmlns="Book/1">'), 
                   '</MediaClass>', 
                   '</Book>')