If your want to replace text in a TEXT or NTEXT datatype.. and your column is less than 4000 characters then you can use the below.
UPDATE DynamicPages
SET Content = CAST(REPLACE(CAST(Content as NVarchar(4000)),'XYZ','AAA') AS NText)
WHERE Content LIKE '%XYZ%'
But I would recommend the below query as some of the column's might be more than 4000 characters, FYI this will only work from SQL Server 2005+
UPDATE DynamicPages
SET Content = CAST(REPLACE(CAST(Content as NVarchar(MAX)),'XYZ','AAA') AS NText)
WHERE Content LIKE '%XYZ%'
