convert update field second to time
Assuming your are using SQL Server database, you can use CONVERT function with style 114 to get the time portion from your seconds value in integer. Please refer to following code implementation.

Declare 
     @FromTime int
    ,@ToTime int;

Set @FromTime = 20520;
Set @ToTime = 39360;


Select Convert(varchar, Dateadd(s, @FromTime, 0), 114) as FromTime;
Select Convert(varchar, Dateadd(s, @ToTime, 0), 114) as ToTime;


To update all the records, if your seconds fields are int or any numeric type you will require two additional time fields in your database and execute the update statement like below.

-- 1) if the fields are int type use following
Create table T1
     (FromTimeInSeconds int
    ,ToTimeInSeconds int);

Insert into T1 values (20520,39360)

--Alter table to add time fields
Alter table T1 add FromTime time, ToTime time;

Update T1 
    set FromTime =  Convert(varchar, Dateadd(s, FromTimeInSeconds, 0), 114),
        ToTime =  Convert(varchar, Dateadd(s, ToTimeInSeconds, 0), 114);


If your seconds fields are char or varchar type you can update the same fields with statement like below.

-- 2) if the fields are character type use following
Create table T2
     (FromTime varchar(20)
    ,ToTime varchar(20));

Insert into T2 values ('20520','39360');

Update T2
    set FromTime =  Convert(varchar, Dateadd(s, cast(FromTime as int), 0), 114),
        ToTime =  Convert(varchar, Dateadd(s, cast(ToTime as int), 0), 114);


Please find a sample implementation of both the cases in this Fiddle.