How to solve subquery returned more than 1 value in strored procedure?

Subquery returned more than 1 value. This is not permitted when the subquery follows =, !=, <, <= , >, >= or 
when the subquery is used as an expression.

UPDATE [dbo].[AD_Staff_Attandance] SET TotHours = 
(SELECT [dbo].[GetTotalWorkingHours] (DateOut,DateIn) AS TIMEWORKED_OUT_IN 
FROM [dbo].[AD_Staff_Attandance] )

ALTER FUNCTION [dbo].[GetTotalWorkingHours]
(
    @DateFrom Datetime,
    @DateTo Datetime
)
RETURNS DECIMAL(18,2)
AS
BEGIN
 
DECLARE @TotalWorkDays INT, @TotalTimeDiff DECIMAL(18, 2)
 
SET @TotalWorkDays = DATEDIFF(DAY, @DateFrom, @DateTo)
				    -(DATEDIFF(WEEK, @DateFrom, @DateTo) * 2)
					   -CASE
                                    WHEN DATENAME(WEEKDAY, @DateFrom) = 'Sunday'
                                    THEN 1
                                    ELSE 0
                                END+CASE
                                        WHEN DATENAME(WEEKDAY, @DateTo) = 'Saturday'
                                        THEN 1
                                        ELSE 0
                                    END;
SET @TotalTimeDiff =
(
    SELECT DATEDIFF(SECOND,
                   (
                       SELECT CONVERT(TIME, @DateFrom)
                   ),
                   (
                       SELECT CONVERT(TIME, @DateTo)
                   )) / 3600.0
);
 
RETURN  (SELECT(@TotalWorkDays * 24.00) + @TotalTimeDiff)
 
END