Table column update.

[cicatrix]

As I understand you'll need to add a new record to the table, not update the first one:
Date....Desc...............Debit...........Credit........Balance
31/12....Outstanding Balance..............................7000

To do this you can create a stored procedure, something like this

DECLARE @StartBal Money,
        @TurnOver Money,
	@EndBal Money;

    Select @StartBal = SUM(Balance) FROM [CASH BOOK] WHERE Date='1/1/2011';
    Select @Turnover = SUM(Debit) - SUM(Credit) FROM [CASH BOOK];
    Select @EndBal = @StartBal + @TurnOver;
    Insert Into [CASH BOOK] ([Date], [Desc], [Balance]) VALUES ('12/31/2011','Outstanding balance', @EndBal);
	
    RETURN