You are going in wrong direction. What you need to update is not the Web page, but server-side data associated to each customer profile and any per-customer data. Typically, it would be a data in some database. As to the pages, they should be generated dynamically based on some ASP.NET pages parametrized with server-side code embedded in these pages.
How to achieve that? By learning ASP.NET, which is shown in your question tag. I'm not joking. Essentially,
the whole ASP.NET technology is devoted to the problems like the one you formulated. If you did not realize it so far, it means that you are not
really using ASP.NET and need to learn it. In case of using a relational database for data storage (by far, the most typical choice), also ADO.NET. So, here you go:
http://en.wikipedia.org/wiki/ASP.NET[
^],
http://www.asp.net/get-started[
^],
http://en.wikipedia.org/wiki/ADO.NET[
^],
http://msdn.microsoft.com/en-us/library/aa286484.aspx[
^].
Also, this CodeProject article will help you to get started with ADO.NET in no time:
Using ADO.NET for beginners[
^].
—SA