This is an old question, but I needed to do this today. The best I can come up with is to write a query that generates SQL code that can be edited in the query editor – it’s sort of lame but it saves you copy/pasting stuff.
Note: you may need to go into Tools > Options > Query Results > Results to Text and set the maximum number of characters displayed to a large enough number to fit your XML fields.
e.g.
select 'update [table name] set [xml field name] = ''' +
convert(varchar(max), [xml field name]) +
''' where [primary key name] = ' +
convert(varchar(max), [primary key name]) from [table name]
which produces a lot of queries that look like this (with some sample table/field names):
update thetable set thedata="<root><name>Bob</name></root>" where thekey = 1
You then copy these queries from the results window back up to the query window, edit the xml strings, and then run the queries.
(Edit: changed 10 to max to avoid error)