(UNIX_TIMESTAMP(s_out) - UNIX_TIMESTAMP(s_in)) / 3600
) AS hrs
But, most databases have a lot of date and time manipulation capabilities. Another one of many ways
SELECT HOUR(TIMEDIFF(s_out, s_in)) AS hrs
If you wanna update the whole table
SET hours_today = HOUR(TIMEDIFF(s_out, s_in))
The column seems a bit redundant though, because it stores information that already exists(just needs to be calculated).