I never really knew that you could retrieve a customer from Dynamics, update something on the customer, and then update the customer in Dynamics. But you can...
There article is a complete set of sample code on how to retrieve a customer and then update a customer in .NET.
If this is new to you, please go through the example on how to retrieve a customer here:
http://dyndeveloper.com/articleview.aspx?ArticleID=172
Before doing this. That entire piece of code is used in this code.