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 = '<yourNewValidXML/>'
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 ( '<Users>
<User Name="Bob"></User>
<User Name="Mikhail"></User>
<User Name="John"></User>
<User Name="Sue"></User>
</Users>' )
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.