Create a class library - BusinessLogic. Add a reference of it in UI layer project.
Create a class library - DataAccess. Add a reference of it in BL layer project.
Now, from your UI, use the object model and pass on to BusinessLogic project class. This class is a Business logic class. Do the changes as per your need here.
Now, pass on the changed data from business logic class to dataAccess project class. In this class, use ADO.NET and pass on the needed values to Stored Procedure.
For getting back data, it will be transferred from DA to BL and then BL to UI layer.
Have a look at these, explaination with samples:3-tier architecture in C#
]3-Tier Architecture Examples
]3-Tier Architecture in ASP.NET with C#
More sample links provided in this answer: Beginner Projects for Visual C# with VS/SQL 2005