Simplest way to edit single entry in XML column?
If you just need to replace the whole piece of XML in once, then you can do a normal UPDATE, eg something like this:

UPDATE yourTable
SET yourXML = '&lt;yourNewValidXML/&gt;' 
WHERE rowId = 1


If you need to edit individual attributes or elements then you can use the .modify method of the XML data-type in SQL Server to update single values.  Here's a simple example to get you started:

DECLARE @t TABLE ( rowId INT IDENTITY PRIMARY KEY, yourXML XML )

INSERT INTO @t ( yourXML )
VALUES ( '&lt;Users&gt;
    &lt;User Name="Bob"&gt;&lt;/User&gt;
    &lt;User Name="Mikhail"&gt;&lt;/User&gt;
    &lt;User Name="John"&gt;&lt;/User&gt;
    &lt;User Name="Sue"&gt;&lt;/User&gt;
&lt;/Users&gt;' )

SELECT 'before' s, DATALENGTH(yourXML) dl, yourXML
FROM @t
WHERE rowId = 1

-- Update one attribute
UPDATE @t
SET yourXML.modify('replace value of (Users/User/@Name[.="Bob"])[1] with "wBob"')
WHERE rowId = 1

SELECT 'after' s, DATALENGTH(yourXML) dl, yourXML
FROM @t
WHERE rowId = 1


If you need more help, please post a small sample of your XML and your expected results.