convert update field second to time

I have 2 fields ( about 1000 records) name FROMTIME and TOTIME. Both fields will have time in second (for example FROMTIME=20520 and TOTIME=39360). I would like to convert and update these fields to time format (for example FROMTIME=20520 becomes FROMTIME=05:420:00:00 and TOTIME=39360 becomes TOTIME=10:56:00:00) Thank you