Problem:
You are adding from CheckInCheckOut
at the end of the UPDATE statement.
try this:
update CheckInCheckOut
set CountHours=ROUND(SUM(DATEDIFF(ss,'2014-05-15 00:23:04.580','2014-05-15 00:25:04.580') / 60.0 / 60.0), 2)
Here is UPDATE syntax:
UPDATE table_name
SET column1=value1,column2=value2,...
WHERE some_column=some_value;