Why are you trying to update the primary key? Having a column that acts as both primary key and foreign key is a strange choice? I would suggest that you need to separate these fields out so that you have a separate FK and PK, then when you update your record you can update the foreign key without impacting the PK.
Saying that, you may have a reason that I can't see (and it would be the height of arrogance of me to assume I knew why you were trying to do something), so if you could explain further why they are the same field, we may be able to suggest alternatives.
[Edit]
I'm sorry to say, but your design has a fundamental problem - you really shouldn't use the same field for both. The best option would be to consider splitting the FK apart from the PK.
If you can't do that for some reason, you are going to have to use a stored procedure to perform the update. In your stored procedure, you need to disable the constraints using
ALTER TABLE tablename WITH NOCHECK CONSTRAINT ALL
Update your PK and then reenable your constraints.
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