Stored Procedure for updating dynamic columns in a table

I have a table with a lot of columns around 30ish

Now I want to create a stored procedure that will update a specific column to a new value.

I the @columname and @newvalue to be the parameters, and an addition @itemId for the WHERE clause so it will update that specific row/data only.

here's my Stored Procedure

USE [db]
GO

SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
ALTER PROCEDURE [dbo].[usp_UpdateData]

		@itemID int,
		@ColumnName	varchar(50),
		@newValue nvarchar(50)
	AS 
	BEGIN
	   Update dbo.ProjectAllocation
	   Set @ColumnName = @newValue
	   Where itemID = @itemID
	END

When I pass the following

@ColumnName: UserName

@newValue: NewUserName

@itemID: 1

it doesnt update row with itemID = 1.

How can I do this?

----------------------- Sharepoint Newbie